my jury's still out on this one-- the fact that he was playing with his father's card is pretty damning; it means he was unequivocally ineligible. Meanwhile, the casino also acted pretty reprehensibly. yikes.
Playing with a card that is not yours should have no bearing on whether he won the jackpot or not. Only if the machine stated that rule on the machine or posted elsewhere in the casino. People are always leaving their cards in the machines. (forgetting them). Cards are only used to track play.
I dont know what the rules are so unless its forbidden in writing i doubt they can whine about it