Had a discussion with the wife...

Does the new guy know that she cheated on her ex with him? If so then just let them be together and then split up when she cheats on him. If you know that your girl cheated on her dude to be with you, then you have to know she's going to cheat on you to be with another guy. Thats the risk you take with it.

Now if he has absolutely no idea about it all, then thats another story. If he seems like a stand up guy then you have to tell him what the deal is.