Nothing. If they are not meant for each other, they will break up. If they stay together, you and the guy aren't meant to be together. If you try to break them up, you risk being seen as a homewrecker or just plain mean. Whatever needs to happen will heppen, and it really will all work out for the best. Just go on with your life and remember that men aren't always the road to happiness...in fact, they rarely are.