OP I think you need to ask yourself some brutally honest questions, all starting with “Why?”
Ask yourself why you didn’t want to tell his wife and wonder all this stuff when you were having a great time, the affair was in full swing and you knew you were actively choosing to play a part in her betrayal. The dilemma didn’t occur to you then. Why?
Ask yourself why you didn’t want to tell her when it ended, but secretly looked her up on social media instead to try to glean information. The dilemma didn’t occur to you then.
Why?
Ask yourself why, when it started up again, you still didn’t want to tell her and the dilemma didn’t occur to you then.
Why?
Ask yourself why, now that he’s withdrawing from the affair, if you still have no intention of telling his wife, you have discovered a huge dilemma situation that was there all the time and you are posting on Mumsnet to see if women would rather know if their husband is having an affair.
Why now?
Why it only looks very much like a dilemma to you now, is the really big question.
You maintain however, that to tell or not to tell is absolutely not a dilemma, you would never tell.
So also ask yourself this: why risk a flaming here and why do you need anybody’s opinion?
Unless you are a journalist researching this topic for an article, or lying to yourself and/ or us about your intentions and are actually trying to get validation for telling her and blowing her life apart, (yes , I know he did that) your motives for being here and asking don’t add add up to me.
Without risking getting a caning here you could have given this a quick Google and tons of stuff appears from elsewhere already written.
If an OW posts on here she usually is asking for advice, they don’t show up and risk a flaming just for a musing or a ponder which risks a great deal of hostility.
Ask yourself this question:
If I’m really never going to tell, why on earth have I outed my OW status on Mumsnet in asking a question it would be safer and easier to get answered elsewhere?