I need some advice

So I have this friend. We’re really good friends and tell each other everything and there’s one girl that we know that is really not a good person and we both don’t like. We both say we don’t really like her and what she does. But one day my friend just started hanging out with her more and more and now she will barely talk to me without snapping at me for a random reason and just acts so different and I don’t get why. I’m one of the nicest people to her. I don’t know what to do?