I don't know what exactly happened. Neither Harry nor Draco wanted to talk about it.

If I were going to guess -- I would guess that Harry cruciated her, but because someone else said he had to, or because he thought if he did, that would be the end of it and nothing worse would happen. Because he looked like someone who'd done Dark Arts, but absolutely not like someone who'd enjoyed it. He looked like he hated himself, and not like he had succeeded in convincing himself that she deserved it. Does that make sense?

Draco was trying really hard to be pleasant at the party but he looked furious at the whole world, when he let it slip.