"I know a lot of people would have said "do her, your M is done, you deserve a little love""
I doubt it.
Of course he has no business having either the emotional affair he is currently having nor the looming physical affair. You indicate nowhere that he is either separated or planning a D. In what way is his M done? He IS cheating on his W already, he needs to stop.
BTW, the same still goes for you. As long as you have zero intention of ending your M and continuing to have W as your primary life partner, you have no business getting into emotional or physical affairs with women other than your wife...
