It never ceases to amaze me how people think. Just step back and review for a second...you're having a full on affair with a married woman, and you're still screwing her daughter at the same time. The married mother has sex with her husband, and the reaction most have is, "she cheated on me"
We the audience know John is a POS that deserves a slow and painful death, but Nicole doesn't...her options are to stay in the lifestyle she's accustomed to, or throw it all away for a kid who's only work experience is working for her husband. Sure she has been told the truth of things for the most part, but that's a big pill to swallow for a woman that has been willfully ignorant most of her life.
From Nicole's standpoint, John doesn't abuse his family, and until the MC started to raise some red flags to John, he leaves them alone for the most part. He just focuses on making his bank account swell, and is always gone from the home on business as far as she's concerned. I'd be willing to put money on the vast majority of married woman would look at Nicole's marriage with envy.