Can the biological mother take children away from non-biological father-figure when he is the primary caretaker?

My friend is going through a divorce because his wife committed adultery and abandoned the family. His 'wife' signed away her parental rights in court, but is now threatening to take the two boys away, whom my friend is not the biological father of. If she has signed away custody, does she have the right to try to take the boys because my friend isn't their paternal father, and even though he is the caretaker? She lives with her boyfriend at his parent's house, and has no job. She pays for the car insurance on my friend's car because of his driving record, but he still pays for her cell phone and for her gas to come see their kids once a month. She is threatening to stop payment on the insurance as well as take the kids. What can my friend do?