It's seldom a true either-or situation. There is a wide range of possibilities besides the two you suggested.
For example, a man can reject a dreadful spouse and at the same time take up with a younger, fit goodlooking woman.
Some people have an almost pathological bias against goodlooking women. Don't hate them 'cause they are beautiful. It's entirely possible for a man to find a very attractive romantic companion that is a woman of substance. I know - I have one in my life right now.
The fact that a man takes up with an attractive woman, in itself, is no reason to cause your respect for him to plummet to zero.
Living by your values doesn't mean living by your lust. I just hope they tried everything to save the marriage first, and it wasn't HIM CHEATING that broke them up. I'm sorry, but he is known around here as a moral example, and I would be disappointed if he didn't at least behave like a mensch. But I do not know the story. I could be way off base.