Riso hediondo pode ser resumido como "caia e pule a sua vez"
Como você observou, o alvo não pode realizar nenhuma ação enquanto ri.
Mover é uma ação (PFSRD):
The simplest move action is moving your speed.
Então está de pé (PFSRD):
Standing up from a prone position requires a move action and...
Dar um passo no pé 5 ("mudar" na linguagem 4e) também é uma ação, embora estranha (PFSRD):
Miscellaneous Actions
The following actions take a variable amount of time to accomplish or otherwise work differently than other actions.
Take 5-Foot Step
You can move 5 feet in any round when you don’t perform any other kind of movement. Taking this 5-foot step never provokes an attack of opportunity. You can’t take more than one 5-foot step in a round, and you can’t take a 5-foot step in the same round that you move any distance.
You can take a 5-foot step before, during, or after your other [emphasis added] actions in the round.
Ataques de oportunidade não são, na verdade, ações de nenhum tipo (nem mesmo ações imediatas), tanto quanto eu sei, então o alvo pode assuma ataques de oportunidade por RAW (PFSRD), embora este GM considerasse uma supervisão e proibisse os AoOs também.
An attack of opportunity is a single melee attack, and most characters can only make one per round. You don’t have to make an attack of opportunity if you don’t want to. You make your attack of opportunity at your normal attack bonus, even if you’ve already attacked in the round.
An attack of opportunity “interrupts” the normal flow of actions in the round. If an attack of opportunity is provoked, immediately resolve the attack of opportunity, then continue with the next character’s turn (or complete the current turn, if the attack of opportunity was provoked in the midst of a character’s turn).
A parte de não estar desamparado parece estar lá para esclarecer que o alvo não leva defensiva sanções além daquelas impostas pela propensão (PFSRD):
The character is lying on the ground. A prone attacker has a –4 penalty on melee attack rolls and cannot use a ranged weapon (except for a crossbow). A prone defender gains a +4 bonus to Armor Class against ranged attacks, but takes a –4 penalty to AC against melee attacks.