Maybe it was the stress. I don't know. He made a selfish decision and he changed and acted selfishly. It is good it only lasted four months. For your M sake and for his sake, he needs to be honest with himself of why he did what he did.
Keep in mind he is not a victim, but he used SOMETHING as an excuse. He needs to identify what that thing or combinations of things were. That way he will not go that path again. It may not be an affair, but it will be something else.
