Terrain is your friend

Take Detect Thoughts spell:

The spell can penetrate barriers, but 1 foot of stone, 1 inch of common metal, a thin sheet of lead, or 3 feet of wood or dirt blocks it.

Seek Thoughts - same limitation. Also, most areas of effect stop at obstructions. I don't have access to 3.5 monster manual anymore and Illithids are not open content, so I cannot check exact wording of the blast, but it is only reasonable to say (house-rule, if needed) that hiding behind a corner makes you immune to the blast, especially if said corner is made of the right material. If your players know that, they should be able to alter their strategy and not face Illithid directly. Luring them in place where he can only target one of the PC at a time and shooting it from distance larger than his blast would do the trick.

To use this method, as a DM it is your role to make sure players know it is an option (as their PC probably should know), and to provide terrain properly suited for the purpose. You should also play on Illithid arrogance, so it is not cautious.

I have successfully used terrain to help my players in fight, letting them hide their weaknesses and play their strength.

Spell like abilities can be disrupted

As said by WotC here. You are using 3.5 monster, so it should have 3.5 penalties to his abilities to stay balanced. Make sure your Wizard and Bard realize that (from the second part of WotC article):

In most cases, one can disrupt a foe's spell-like ability in exactly the same way one disrupts a spell. For example, a creature's opponents can ready attacks to disrupt its spell-like abilities, and if they hit the creature while it uses a spell-like ability, they may cause the ability to fail. The DC for the creature's Concentration check is exactly the same as it would be if the creature were casting a spell.

Pathfinder roll20 site seems to agree on that:

A spell-like ability can be disrupted just as a spell can be.

So again, strategy! Let someone shot your Illithid in the face when he starts his blast. If needed, fudge his concentration roll to make him fail.

This is something I've used as a player, thanks to friendly reminder of my DM.

Spell like abilities can be dispelled

You say about mind-controlled orc minions. Bard of 7th level should be able to cast Dispel Magic. Wizard 5th, too. Both should be able to use Spellcraft or Knowledge or Sense Motive to recognize mind control. They could, and probably should, try to break this control. Pretty much all of the D&D lore shows that creatures that used to be enslaved by Illithids hate them when they are freed, so it would turn opponents into allies, and give Illithid other targets than the party.

As a DM, it is your duty to call appropriate skill check and then tell your players what their characters are supposed to know already. Then, they may adjust their strategy. But it is your duty to give them info, or at least to give them a chance to get that info, to make informed decision.

Bottom line: it is all strategy

Bard attacking and Wizard using AoE damage spells in their first rounds does not look like a winning team in my dozen or so years of play and DMing. Teams that have used Wizard as battlefield control (Walls, anyone?) and Bard as support (higher saves with inspire Courage or buff spells, please?) were what made parties successful against "boss" monsters. For some monsters, blasting and slashing them in the face just won't cut it. Illithids are suposto to be formidable enemy and PCs are suposto to be scared about loosing their minds when fighting them. They met first one. Now, they should plan what to do if they will meet next. And you should make sure that they will actually plan a strategy. And if that strategy is sane, make sure they'll get what they need to succeed, like spells in Ye Old Magic Shoppe, and decent terrain with a lot of corners.

Re-balance the creature stats

The 3.5's version of the Mind Flayer (TM) has very strong abilities for their CR (8), and even if you convert the creature as is to Pathfinder (which should be CR 7-ish), you have to tone down some of their abilities, especially Mind Blast, which was written in the first Monster Mannual and still had the mentality of save or die on certain creatures (ghouls, basilisks, cockatrice, mind flayers, beholders, etc).

So, whenever you see abilities like that, such as "if you dont make this save you are dead/out of combat long enough to be considered dead", think about ways to balance and weaken the ability a little to pathfinder standards. For instance, the Pathfinder vampiro is still deadly (1d4 rounds of paralysis), which is considered an unbalanced creature for their CR, similar to the mesuda (CR 7). But cockatrices have been weakened because of situations like that, and now reduce your dexterity on every attack, petrifying when you reach 0 dex, instead of instantly petrifying the character permanently. A basilisco is still very deadly, but you now can revert the petrification using a dead basilisk's blood. Which means that, other than a TPK, that petrification won't "kill" the petrified characters.

However, keep in mind that the Mind Flayer is a very weak martial creature, relying on dominated creatures to fight for them, weakening their stun could make them less of a threat and more like a piñata. They have that ability for the sole purpose of affecting strong martials (low will mostly) so they can handle the group's casters on their own. The goal here is to make sure the encounter isn't above the PC's ability to handle it.

From what you described, the ability had the intended effect, it removed 2 out of 5 enemies from the fight temporarily.

Maybe the number of orcs was just too much for the encounter. Consider that Mind Flayers are CR 8, against a party of level 5, so the creature is APL+3, which is a very difficult encounter. Common orcs, even with a low CR (1/3) are also known for being dangerous due to their high crit rate (18-20) and high average damage for their CR (9, ignoring crits).

Don't beat dead horses

This bad taste you guys felt was due to the orcs focusing on a character that couldn't contribute to the fight anymore, instead of fighting those who were actively fighting. This is sempre frowned upon in my games, players having both having been removed from the fight and getting killed slowly without any chance of reacting.

This has happened to me when they fought a band of ghouls (6 or 7) when they were about level 4 or 5. The ghouls got lucky in the first round and managed to stun all but one party member, they couldn't pile into the single survivor and started attacking those who were already paralyzed, killing the group's barbarian in the process before the remaining PC managed to force them to flee.

We had a short discussion about that encounter and they all agreed that it felt unfair that the "strong and powerful" barbarian to be eaten down during the fight without any retaliation, while there was a ranger shooting arrows at the ghoul's heads. And I agreed with them, but they also agreed that there wasn't anything else that those extra ghouls could do due to space limitations in the fight.

Sometimes it just happens

I mean, what are the odds that the character with the best saves in the game to fail their will save when it is the most important save they have to make in their adventuring career? I know, right? 100%.

Yes, it happens cada vez. I had a paladin with +12 save fail a DC 14 dominate effect against a vampire spawn in a Ravenloft game, he was the only character dominated and had to fight his own allies for that encounter. Not only it's terrible to be one man down, but that man joined the enemy team, and he had the archetype that increased the AC for all allies around him (so, the vampire spawn) and has strong saves and could resist all attempts at casting Proteção contra o mal on him (which he obviously resisted), making it much harder.

They were level 2 at that time fighting a single CR 4 creature, which should be a difficult encounter, but in no way something impossible. They managed to handle it, but they had to restrain the paladin before they could do anything to the vampire.

So, don't worry about it too much. That is also part of the game. Even if the character died, it should have been a memorable moment for everybody involved.

Roleplay the character having a mind battle while stunned

This is an idea that I saw in the Neverwinter MMO, in one of the player created content dungeons, and it's the best experience I had in that game. The player character is hit by a mind blast and automatically fails their save, because the Mind Flayer is much stronger than you are. Then you are teleported to a mindscape dungeon and fight mind constructs there, facing the boss Mind Flayer at the end on a Battle of Wills against a huge-sized, beefed up, muscled, mind flayer that attacks you fisicamente (yes, this all happens inside your own mind). I won't spoil it, but you end up defeating him and returning to reality to see a, possibly dead, Mind Flayer no chão.

To me, that idea was fantastic, and if I ever run one of those creatures again, I will have no shame in stealing it. It felt great in a game, and I believe it would feel even greater on the table, because mindscapes are a thing in Pathfinder. I have used it several times and every time the players loved it.

Personally, I think it is the responsibility of the players to come up with a decent strategy. That is part of the challenge and expected. Your encounter looks like it is in character for a mind flayer. So I wouldn't change anything on your side. But here are some ideas for you and your players.

1. Protection from Evil helps against the secondary abilities of the MF. If they dont have that, allow them to purchase it.

2. Elementals can not be stunned. Undead are practically immune to the MF but your paladin might not like it. Give your players the opportunity to purchase scrolls or other summoning items. Some bodyguards go a long way.

3. Use ranged weapons and spread out. The blast is a cone so move aruond the monster. MF are not very durable. A smiting pf paladin with a bow will shred it.

If you really think this is necessary, maybe hava a van-helsing-type npc communicate these ideas to the players. Be.... not subtle.

Atenciosamente