I'm not married so can't say much about that and I've read this before but in real life someone like that guy must be really ignorant to not noticing it . i mean when you live with someone for 10 years, you probably understand every change in them either it be a divorce situation or some other situation .
Actually that part is not that unbelievable. When people get involved in extramarital affairs they can be extremely self centered and blind to what their partners are going through. Some may realize it later some never even realize the extent of the damage caused.
Evey small argument ever occurred will be remembered as a major point of a disagreement, every minor short coming would look like greatest draw back they cannot live with it anymore, everything else takes all the energy and time away in the blame game of why the relationship didn't work. Everything related with them becomes stressing . At the same time the new one is still fresh and exciting and the new partner is the new ally while they need to deal with the vamp/villain aka the old spouse.
It's always a mess. Even if someone is able to build the relationship back after ending an affair it would never be the same as the old one. It may work out but the conditions and expectations would be different and it takes a really longtime before one can really trust the other person of the same kind.