That is kinda mess up. So in this case, the mother and father abandon this kid and he fend for himself, finally made a decent living, married, has his own kids and now his estrange mother wants to sue him for payment? What about all the time when he needed her and they left him alone at 15!?!??!!?
There got to be a case where he can have the option not to pay since his parents didn't really "invest" in this kid.
I think it should be case by case, my parents DID spend a ton of money on me and my education, when they can't pay for it, I got a loan and finish paying it myself, but they did provide me during hard time and such, so I personally would support my parents when they need me, but this kid shouldn't have to or at least get reduce amount in terms of years of at (0-15) and that is it.
I know it sounds cruel, but to me, it would be like a child put up for adoption later won the lottery (say 50 millions) and parents discover it and want a cut being their parents.... sorry chief, that is a no go in my book.