While I personally don't endorse or approve of cheating, based solely on how the story played out, 'Senpai' (a.k.a. Baka Husbando) brought this development on himself. Also, Abe-san is by no means completely innocent or virtuous in the matter.
If you prioritize a promise made to a friend, over a promise you already made to your wife, you're priorities are totally fucked. Even if you weren't cheating, you're still fucked.
Fragile, huh. Sometimes I feel like Hentai is trying to tell us not to get married because everyone will cheat on us, or something.
Then again in this case it's the idiot husband's own fault, so I suppose the moral is don't cheat? Or perhaps I'm thinking too much.
And in fact Abe blackmails her husband to lie to her so that she thinks he cheats and the guy is able to seduce her while the husbands lies tied up in Abe's flat...
Wouldn't that require that they aggred to that and both know that they have sex with different people?
I would call what they have a bad relationship and nothing more...