Well, here's a possibility:
* he's a weakling who lives in his mind rather than reality. Has leaned on others all his life, forced them to clean up his messes. He never accepts responsibility - always someone else's fault. Has only one thing going for him - gift of the gab, which comes easy for him because he's a self-absorbed fantastist and manipulator, user, etc.
* he's hopeless in bed, so his spouse pretty much gave up on that side of things anyway. Sir Galahad had proven himself a tadpole, not even a toad. Threw herself into her kids and plans for their future with herself as sole parent - and was actively pursuing same. While she'd initially been hurt and wounded by betrayal, her intelligence pulled her through. She saw him as a little boy, could see the years vanishing with no improvement in sight. And realised his betrayal/extra marital dalliances were more symptoms of his innate spite, resentment, jealousy, insecurity rather than of his libido. So she rose above the idiotic nonsense with the other-woman - even pitied her
* never in a million years would she stoop to snooping on her idiot spouse and his deluded lover. She would not give them that -- would not give that to any of them
* he's a gutless wonder. He knows everyone knows it. He got himself into major problems. Money. Promises and assurances he had no way of honouring. As usual, he figured someone-else would fix them or offer him a way out -- probably because they always had in the past. Problems compounded because others didn't have the luxury of living in la-la land like him - they had to eat, pay bills, etc.
* eventually, someone with a lot to lose was forced to take matters into their own hands. There were probably several tearing out their hair courtesy of the idiot by this stage. One or more got together. It went too far, went wrong. They tried to save their necks. Like fools, they asked the idiot to do ONE SIMPLE THING. And he stuffed up again and will destroy the lot of them as well as rendering innocents 'collateral damage' into the bargain
(all my opinion or possibly a rough draft for a book which will never be written)