If he A. Isn't the biological father and B. Never acted as the child's father (meaning didn't make the foolish decision of marrying a woman with children) - all he has to do is file for the divorce and prove he isn't the parent. He's not on the hook forever, just until he gets it cleared up. Maybe 6 months.
The price he pays for never getting the divorce finalized...and for marrying a bad woman in the first place.
The price he pays for never getting the divorce finalized...and for marrying a bad woman in the first place.