If I had to guess how this story would play out, she reluctantly agreed to it as it would make her husband happy after he heavily pressured her into doing this for him. She'll then realize she's happier with the other guy than her husband who doesn't pay attention to her and had no issues sacrificing her for his own happiness. Whether it's a happy ending for anyone would be dependent on if the other guy is just using her or not.
This of course is built on hentai logic, and we obviously know this isn't how it'd play out in real life.