showing cards questiom

Just wanted soem opinions on something that came up at my home game. Two players left in Holdem player A bets $20, Player B calls. Player A turns over both cards and shows a pair of queens. Player B turns one card over , a king, which matchs a king on the board giving him a pair a kings. Player B states that he doesn't have to show the other card.

Whats the ruling on this. I know its a home game but I say the rules of poker state you have to show both cards when calling. The TDA doesn't cover this