Let your backbiting friend rot in hell
When a person backbites, his all good deeds are washed away. You should encourage him in backbiting and let him go to hell. By this you will end up with good deeds only.
Assist them in recovering
You should help them recover from this bad habit. Remember that backbiting is a really bad habit and is an unmoral act in our society. Avoid fighting and help them recover from this problem.
Learn the lesson
After you find out a rat among your friends, you should avoid telling your secrets to him. Do not share your secretes and other personal stuff with him. You should keep your mouth shut when staying with him.
Keep in mind that these rumours won’t affect you or your health at all. Remember that a backbiting friend is worse than your enemy. After you are confirmed about the person, you should simply kick him out of your life.
Talk to him
When you find out that your friend is passing false rumours about you, you should confront him directly. There might be a chance a third person is involved who is jealous from your friendship or that person might not meant what he said.