He would probably say that he was unhappy in the marriage regardless, so the issue of another woman, whenever she came on the scene, is just a detail. If he was happy in our M, he wouldn't have been open to someone else.
I think everyone is unhappy in their marriages at different points, nobody is 100% happy 100% of the time. If someone suggests they had an EA/PA because the M wasn't happy, I think that's a bunch of bs. Honest communication and/or MC would be the real solution if that was actually true.
