That's not how I'd say it, if I were to meet with her. (Because that really does sound high-schoolish. It's already implied that people have the right to be friends with whomever they choose.) I would let her know that "I feel hurt because my long-term best friend has been listening to someone she knows doesn't have my best interests at heart" or something to that effect. With certain words or terms you risk her becoming defensive and then any meaningful dialogue will break down. Just let her know how this has made you feel - without saying anything about the future of your friendship. The onus is on her to either fix it in regards to you - if she wants to keep the friendship - or be cut loose, because you have the right to not feel hurt by a friend. This way you've set the stage for her to put up or shut up, so to speak. If she continues down the same path, then you have your answer - because she knows you're hurt by this and if she continues anyway, then it's clear she doesn't care. If she does make changes, then you can see how it goes. Good luck. Personally, if a friend did that to me, the writing would be on the wall that she wasn't so good a friend in the first place. We've probably all had that realization at one time or another, and it does suck, but life goes on. Find people that actually care about you, your feelings, and your trust.