Not understanding the math pretty much undermines a significant portion of your post. He didn't marry his wife the day they met, but rather four years after that. Add four years of marriage and you have eight years total.
It follows then that she cheated on him in that four-year dating/engagement period. I won't speak for anyone but myself, but in a relationship in which there is presumably the kind of commitment and trust upon which a couple eventually bases their decision to spend the rest of their lives together, such a significant breach of that trust is kind of a big deal.
I would be super pissed. I know a lot of people who wouldn't like it if the person they were dating fucked somebody else. Come to think of it, I don't know anyone who would be cool with that.
That he also deceived her by checking her email is noted. But he didn't stick his dick inside the computer. She likely isn't worried about him one day running off with the computer or leaving her for it. She isn't wondering whether he truly believes in the concept of marriage.
Not confronting his wife about it directly was a questionable approach, yes, but are you really criticizing him for acting irrationally after making this kind of discovery? A jerk?
I suspect quite a bit of listening and understanding was required of him to ultimately forgive his wife, which he has, and make the marriage work. In fact that takes the kind of understanding a lot of people aren't able to muster.