I would certainly say do not be a part of a triangle. If he wants to go to his ex-wife, then let him go. This will probably be terribly difficult, but it can't be both ways. I hate to say if he chooses to be with her, I don't think you can ever be a part of that life. Boy I hate to say that, loosing some one is so incredibly difficult.

Please though, don't give up on relationships. I know a woman at my workplace that just did this to a man she was dating, so it works both ways. This can happen to anyone.