Well I have actually had two of my close friends turn into backstabbers. The first one was my best friend of two years. We moved in together with another girl in our sorority and that's when the trouble began. My best friend, at the time, was telling me things that our other roommate was supposedly saying about me and telling the roommate things that I was supposedly saying about her. Luckily me and the other roomate sat down and talked about what things we had been hearing and come to find out that none of them were true! So we put two and two together and starting realizing that our friend was pretty much a compulsive liar. Then our lease was up on a house we were living in so we moved to a three bedroom townhouse with my fiance. That's when things got real interesting...let's just say we moved in near the end of January and we kicked her out at the beginning of April. She started spreading rumors, talking trash, and even revealed a deep dark secret of our other roommate. That was fun to be in the middle of!! Funny thing is she was my maid of honor in my wedding and now I pretty much can't stand the sight of her. I saw her a couple of weekends ago at a bar. She wanted to buy me a shot and I just turned and looked at her and said "no thanks" and walked away. I could tell she was pissed but I don't care. Friends don't shit on friends.
My other friend was a new member to our sorority. She was really fun, energetic, and great to be around. Then she got initiated and starting being Miss Gossip. Suddenly if someone looked at her she was stressed. My good friend of five years was really interested in joining the sorority and of course Little Miss Gossip starting telling me that no one wanted her in the sorority, which come to find out was a big lie. Anyway, I'm pissed at her because she caused such a stink about my friend joining and then she went inactive. Now my friend who wanted to join really doesn't want anything to do with the sorority...which is pretty bad because she went to more open events then the active members! We had awards a few weeks ago and there were some unofficial awards that we obviously didn't give out but the girl who went inactive was voted Sister Backstab
Basically all you can do with a backstabbing friend is confront them and tell them that you will no longer tolerate them and their need for attention and drama AND/OR cut the strings on your relationship with them.