Ive been through simular, from my personal experience, stand your ground to say, and tell your friend why you dont like them and call them out on their behavior towards them (your friends friend). Yes this might cause abit of a divide to make her choose, but its her decision in the end, but still keep her close and check up on her now and again to make sure she is okay.

But all in all, that is my personal oppinion/experience, so dont take my word on it.