I know how hard it can be to lose a friend, but if they are really getting to you, you should just stop being friends. This happened to me once with my best friend. I had known him since I was little. He stopped hanging out with me and started hanging out with people that didn't like me.

Eventually she will break up with the boy and will start hanging out with you.

Have you tried talking to her about whats going on and how you feel?