It is a shame your friend had to argue so long. He was right.
Hypothetically speaking though, lets say he was wrong. If he had known the rules prevented him from using the card to "save" his life in the last moment, don't you think he would have used the card as soon as he got it (or was below 9 hearts)? There would be very little reason to hold the card and wait. It would be pretty poor sportsmanship for you guys to take advantage of him not knowing the rules! Especially when there is such a clearly obvious play to be made if you do know the rule.
Hypothetically speaking though, lets say he was wrong. If he had known the rules prevented him from using the card to "save" his life in the last moment, don't you think he would have used the card as soon as he got it (or was below 9 hearts)? There would be very little reason to hold the card and wait. It would be pretty poor sportsmanship for you guys to take advantage of him not knowing the rules! Especially when there is such a clearly obvious play to be made if you do know the rule.