I became friends with a woman I work with. At that time she was going through a break up with a man we also work with.
I told her that me and this man had also been together (nothing serious, just sex).
He lied to her and told her he had never been with anyone else since his ex when clearly he had with me.
They broke up because he'd started a new relationship behind her back. She confronted him about what I'd told her and he said I was lying in order to keep them broken up 
It's also come out that he'd lied about a lot but still denies everything.
They have started talking again and it seems obvious they will get back together.
I feel sad because I feel like I'm going to lose a good friend. This man doesn't like me, he gives me dirty looks at work etc
If your boyfriend hates your friend then .... it's the friend you are going to see less of 