He fell in love with the wrong person I guess, he needs to realize that there is someone better for him and that love is much more beautiful then what he is having right now, you could try to explain him that it could be better and make him understand what he is doing isn’t worth it. But remember that it can lead to a fight between you both, because sometimes a person who is in love with someone is too stubborn to understand it. If he ignores about what you are trying to tell him, then i suggest you should move on, because it will just make yourself upset and hurt your feelings. I know it would be hard, but if you are strong enough, stay friend with him, sooner or later he may realize what kind of mistake he made. I hope I could help you. All the best & stay strong!