I’m afraid you would have no other choice than to tell this girl. I don’t know any of you, so I can’t assume you or this guy’s character. But I will ask you this, if you had a boyfriend and he cheated on you for a friend he's known for over 3 years, wouldn’t you want to know? Cheating is something you should never overlook. This girl is the innocent party here, she doesn’t even know what her boyfriend is up to when he leaves. And this, in my opinion, is very sad. I strongly believe that a cheater should never be protected. He has a girlfriend, God knows how many other people he has treated on her with. In a relationship, both partners must be faithful. That’s what, I believe, makes a strong and long-lasting relationship. I know you love this person, maybe that’s why you’re not seeing what me or many others might see. But cheaters should never be protected. Yes, when this girl discovers that her boyfriend is cheating, there would be heart-wrenching emotional effects. But it would prevent him from doing this again since he was caught. If you decide to stay silent and let this go unseen, then he would feel invisible and go on with his cheating tendencies. Again, I don’t know any of you, so I don’t know your characters. I’m just telling you from my experience and from what I’ve seen time and time again. I have no advice to help you keep this man in your life. If you keep contact with him, then it’s highly likely that you might have other sexual contacts. You must ask yourself this, what would you want someone in your position to do? Take a minute, look deep inside of your heart. Deep, deep, deep down. Instead of thinking about your relationship with this guy, take a minute and put yourself in this girl’s shoes. I know you are a smart person and I know you would never want someone to do this to you. The decision is ultimately in your hands.