What should I do about this?

It sounds like shes a real idiot, its not your fault that he likes you and not her. Its a jealousy thing. If you want to resolve it, try and talk to her say something like " hey I don't want any drama here or any negative relationships, you don't have to worry about me and ( guy A) nothing will happen". If after that she still is a b*tch to you, then just forget about her. I learned not go care what people think of me, if your liked then good, if someone does not like you then they can go f*ck them selves, and that goes for everyone who takes her side as well.