I would not go so far in the confrontation. I would just say, "Why did you friend ____ again? I thought you had promised not to do that." (Sounding confused). And then listen to what she says. Then you go on and say how it hurts you and it not only impacts your relationship with her, but also with her parents at a time when the family needs to pull together. And again, see what she says. I wouldn't threaten anything, I would just let her know that she is isn't fooling you, and she is hurting you, and perhaps even her mother at this critical time.
