What do you do when you know that your boyfriends' best friend is not loyal to him?

I have been dating a guy for over two years.
We both have our mutual friends that we hang out with periodically. We have had issues in our relationship and everyone is aware and always putting their opinions out there about who's at fault or who's right. My boyfriend and I decided to just not mention our problems to our friends and family.
His friend basically will call or email me and my friends giving his opinions about my boyfriend. But he won't be honest with him. I stopped talking to the friend because I felt that he wasn't trustworthy, and didn't have our best interests at heart. But it's obvious that he wants to add fuel to the fire- for whatever reason. How do I tell my boyfriend without hurting his feelings? I don't want to come between two friends. I am confused...