How do you get a cheating spouse to stop cheating?

A co-worker / friend of mine has a wife that is at least emotionally involved with another man (I don’t really now the extent of the relationship just what he has told me). He says he really doesn’t want a divorce, because he still loves her very much. He asked me this question on the way to work this morning and I told him I would have to think about it. I cant think of anything to say so any honest suggestions would be greatly appreciated.

Huhtala