Why has he had an affair if his marriage was so great
Oh and just to point out that there is such incredible naiveté about thinking people only have affairs because they're in "bad" marriages.
Loads of people, most commonly men it seems, have affairs and then beg, on their knees, to stay in their marriage. Loads of men will admit privately that there's nothing wrong with their marriage but they want some extra on the side. Loads of people will appear to subscribe to monogamy, in order to get the benefits of monogamy and fit into our society...with no true belief in it or requirement of it for themselves.
So the idea that people have affairs because there is something missing or wrong ....totally ignores the nature of a lot of humans, including their lack of integrity, utter selfishness, double standards and entitlement etc.
In the type of case scenario here, some men will dump their spouse/partner when she passes menopause. Some will dump their spouse/partner when they get bored shagging them. Some men will prioritise sexual attraction and their sex life above almost everything else. If they get a solid opportunity with a younger woman, whom they haven't gotten bored shagging for decades; they're off.
Their looming late middle age is another factor in that. They can't face the idea of themselves as middle aged, as a relatively soon to be Grandad, as their life being "over" in terms of the mating game etc. If they get the chance with a younger woman to feel they're still in the mating game, and still relevant, and still "young" ..they'll take it.
Their marriage doesn't have to be bad in any way for all that to mean they exit it when they get the opportunity.
They will make up all sorts of excuses for why they left because "I was utterly bored and unmotivated about shagging my menopausal wife who I've been shagging for decades, I couldn't be bothered with the whole thing, our kids are grown up, loyality and history don't really mean anything to me, I'm aging better than she is, I was able to pull a younger, hotter, non menopausal woman and I can start an exciting new life with her and feel like I have a whole new lease of life, instead of stultifying in my "worn old shoe" life" etc etc ...doesn't tend to make you look very good to other people.
Decent people don't tend to admire men discarding their long term, loyal, devoted, "raised his kids" wives ..... for younger, shiny new ones.
Which is why lots of excuses and usually a history rewrite are necessary. The marriage has to have be "bad", the wife has to have been ABC.
Don't be so naive about that too.