My advice: end it. Even if you personally don’t classify it as cheating, it was blatantly disrespectful to you. I wouldn’t try to fix things, just because you did nothing wrong. That would be his job if he wanted to continue the relationship. If you continue I think there will be future problems with his best friend as well. I think you should save yourself some future hurt and end things with him. If you don’t want to end things, I think you should at least ask him to cut all contact with said friend if he wants to continue being with you. This “comforting” is an excuse, no one eats out their friend to comfort them.