I would have a talk with your BFF and then just totally ignore this other person altogether. Nothing gets to them more than when you act like they don't even exist. If your BFF is really your BFF, she'll do the same.
My 'not really a friend' is being mean to me... She always tells me to shut up, she talks to me like I'm stupid, she never takes me seriously and I think she's stealing my BFF... and when I tell her to stop she tells me to shut up?
That happened to me before, but it all turned out good. First, if it looks like she is trying to steal your BFF, talk to her about it! I did it, and it turns out that she felt the same way! So my advice is talk with your BFF, and if she feels the same way, shut out that other friend! She will only make your life worse, so get her out of it!