Stay away from her. Like higher4hockey said, I would not want one of my friends dating my ex's. And like clever421 said, this is something that could, and I bet would, probably really, really hurt him. And as slipknotpsycho said, she could just be using you. I mean, there can be exceptions, but in this case they hate each other, and in that case, you need to stay on your friends side. You don't need to get involved, but you need to choose who it is you're gonna continue to have in your life and your friend is much better than some pussy.

Also, did you first meet her because she was dating your friend, or was she like someone you met but the only reason you ever saw her again was because she dated your friend? If so, then you shouldn't even see her again after they broke up. Keep away.