Okay, so if I were to roll back the dump, Poloniex would be short about 80 BTC. I have messaged the hacker again, and hopefully he will stick to his word. His actions don't really seem consistent with someone who just wanted to steal BTC--as I said, he left about 35 BTC in his account. Why he decided to make a mess, though, I don't know.

As I don't think he has the XCP to cover the dump, I'm probably going to end up trying to roll back the trades. This might get a little complicated, though, as I didn't see it happen right away and people withdrew XCP after the dump. I'm going to wait until tomorrow (about 12 hours from now) to hear back from him, and then I will most likely start rolling back the trades.