MaxH, you said no player bet, but Christine said "Player A bet and Player B called." After the last betting round, if all players have checked, I believe that the rule for showing a called hand at a players request is allowed. A checked hand is still a called hand.
Dave and Christine,
The procedure for showing a called hand, when the player requesting to see the hand is not involved in the final betting round is as follows;
The dealer will touch the folded hand to the muck (killing the hand), award the pot to the winner and then turn the mucked hand over. Touching the hand to the muck is merely a formality, the surrendered hand is dead. However, if the would be winner of the hand requests to see the hand, it is live.
Christine's example: Player A Bets, Player B Calls. Player B immediately shows his hand and Player A mucks his hand. Then a player not in the hand asks to see the hand. I think Christine was correct in her ruling that the hand should be revealed, however the above procedure needs to be followed so if the hand is a winning hand, they will NOT be awarded the pot. If Player A Bets and Player B calls and Player A mucks his hand, Player B could lose the pot if he asks to see the mucked hand.