If he has ace face up and checks and sees a king he is supposed to flip it over, show his blackjack, and take everyone's money. He instead let them play the hand out as if he didn't have the king.
Minor point of clarity... it doesn't have to be a king... in blackjack, a 10, jack, king, and queen all have the same point value (10), so any one of them paired with an ace makes a blackjack.