Friend who messes up my friendships?

I have this friend. He's like my best friend. The problem is he can b a bit of a *****. Talks about me behind my back. Can b quite influential. I introduced him to some of my friends, and now he is in our group. My other closest friend and I had a massive fight. The thing is.. We weren't even really fighting. Something very small happened. We were all good. Then she went to see my other best friend, and the next time I saw her she was enraged. I know he was talking about me.. He told me. He defs influenced her. I kind of regret introducing my friend to my other friends. Things are now weird between me and my closest friend. She now goes to my other friend for everything.. I hate it. I want my relationship to mend with my friend. Help?