What would you do if this happens to you?

Personally, the man has a child with this other woman so he will always feels a love for her as a mother of his child. However people do move on. It just what vantage point you feel you have with this man. You are not stuck on a tiny island with only this man, and no man is an island so its not as if hes the ground you walk on. Set some ground rules, R.E.S.P.E.C.T. and gauge his reaction.