Algo como "Deixe-o" ou "deixe-o em paz"
Na cena equivalente no livro, ele diz "Deixe-o em paz" (ênfase minha):
Harry wasn't sure what made him do it. He wasn't even aware of deciding to do it. All he knew was that his legs were carrying him forward as though he was on casters and that he had shouted stupidly at the snake, "Leave him alone!" And miraculously — inexplicably — the snake slumped to the floor
Harry Potter and the Chamber of Secrets Chapter 11: "The Duelling Club"
Desde que os primeiros filmes ficaram bem próximos dos livros (para as partes que eles não cortaram, pelo menos), isso parece uma resposta plausível para o filme também.
Naturalmente, seria negligente não mencionar que uma versão do script (enviado anonimamente para a internet, então eu não posso garantir sua autenticidade) tem ele dizendo "Deixe-o":
Lockhart flicks his wand. BANG! The SNAKE flies into the air, HISSES in rage, and slithers straight towards Justin Finch-Fletchley. As students SCREAM, Harry -- oddly calm -- approaches the snake. It rises, fangs exposed, poised to strike Justin.
HARRY (in Parseltongue) Leave him!
The snake looks into Harry's eyes, then turns for Justin.
HARRY (in Parseltongue) LEAVE HIM!
The snake hovers a moment more, then -- miraculously -- slumps to the floor. Harry blinks, as if coming out of a trance, grins curiously at the snake, and offers his hand to Justin.
Harry Potter and the Chamber of Secrets (2002)