Why would a man cheat on a good woman?

How do you know ? Do you know the man are you "friends" with him since you do not know her?

If I were his friend (and not hers) - I would pull his a$$ aside and tell him what an idiot he is. I would say I need to say this and say this once - you are being an idiot - this relationship is destined to not end well. The both of you will be better off if you just break it off. Then let it go. You got it off your chest, but it really is no business of you further.

As an aside my roommate in college started having an affair with an older married man. I did that - I pulled her aside and said I am going to say to this to you once - I care about you and need to say it - you are an adult and can choose to do what you want - but this is wrong and you are just going to end up getting hurt and you are hurting a woman and children you do not know. She kept on with it -- I kept my mouth shut and it all happened as I said.

Sometimes people are just stupid and you cannot do anything about it. I do feel for the woman but you really do not know her -

My sister is a childhood friend of his. She saw him with another woman, she did not want to assume then two weeks ago we both saw him kissing a girl in the back of a bar a town over from where we live.

Great advice, thank you.