Most Helpful Guy
Well it possible he enjoys it. Some people get off on their s/o screwing other guys. But more likely, he's just afraid of being alone. Based on your age this is probably one of his first lovers and he probably finds in more convenient to stage with a cheating girl, than cause a stir and possibly lose his girlfriend and best friend. When you're young you haven't experienced a lot and you tend to think you'll never love anything as passionately as you do in that moment, and nothing will ever love you back as much either.
Most Helpful Girl
The only reasons I can think of explaining why someone’s would stay is that they have low self esteem and think they can’t get anyone else. Maybe they’re scared of being alone and like the company that a relationship provides. Or, the cheater has something material that the other person wants/needs. Say, when you have women married to rich men that they know cheat on them often.