Friend hangs around a not so nice group?

Anyway, this girl keeps trying to take my friend away from me. She acts like a three year old, she crawls up into a ball and starts crying about wanting to "play" with her. I'm a teenager and find this completely immature. But anyway, my friend gets soft hearted and decides that she should defend this girl, and I mean RUDELY. This girl is a backstabber that kind of bullies her into being her friend. How do I just get my friend to get rid of this girl? Also, How can I help her to lose her mean-ish ways when she is around this person if I cannot get rid of her?