While the husband has most of the blame, it's not like the mistress is completely blameless either. She's a willing and knowing homewrecker (in this case).
Honestly, both of the people's career (the husband and the mistress) should have been ruined.
Of course, there are times when people unwillingly become a mistress. Maybe forced by circumstances. Or maybe forced by the guy. Only in cases like that are the mistresses blameless.
Nevertheless, most of the blame lies on the husband and it will always lie on them.
As for your last sentence, I kinda agree and disagree at the same time. I liked the author's stories of Mint na Bokura and Ultra Maniac but I dislike Random Walk and this because most of them forgives cheating too easily.
Why the blame always go on the mistress? Even if she wasn't there, the husband would still be able to cheat with other women. And as he said his girlfriend became more important to him and he agreed to the divorce because that's what he really wanted. And why would he enter another relationship when he's already married? He just can't keep his dick on his pants. This author's stories are mostly shit.