I'm annoyed with my friend for sleeping with a girl in a relationship
Did your friend know she was in a relationship?
He didn't seem to care that she was in a relationship.
I understand why you're annoyed, but you should be careful about getting involved in their relationship drama. Maybe you can talk to your friend about why you don't support his behavior?
I know, but I told him to stop, and the girl's boyfriend attacks me on a social network. I can't put up with this shit.
You don't have to support everything your friend does if you don't agree with it. It's okay to let him know that. But you should be careful about getting overly involved.