Really, your daughter is an adult, and responsible for her own decisions. Effectively, all you can do is make your opinions very clear - just be factual, keep emotion out of it, and make sure that she knows that you will support her. It might just be useful to remind her that actions DO speak louder than words, and his actions have spoken volumes. We humans only indulge in behaviour that brings reward of some kind. Only when that reward (whatever it might be) disappears, or the consequences of our behaviour promise to be unpleasant do we consider changing what we do. For this very reason, this man is going to continue his infidelity.