100% agree with this!
The hand was shown down until the player got the bad news. I would find the pair in the muck, table them face up and run the hand, explaining the reasoning for it is chip dumping. You cannot muck an all-in preflop hand, as it raises integrity issues for the game. Set a presedence that all showdowns should have both players cards face up - then run the board. Both players had shown their cards, so just run the board after locating the 6s in the muck.
Now, if the other player would not have shown his cards and mucked them, I would have called a penalty for the player and explained why. The intent is not to be mean spirited, but to make them understand that this type of behaviour doesn't fly. Start with 1 round penalty -> DQ if they keep doing it. They'll learn quickly. If you would declare the hand dead as many are saying here, you are sort of blinded by one rule alone. Let's put up another example, but this time in a cash game setting:
$1/$2 blinds with a mandatory $4 UTG straddle.
Player A is UTG+1, and for some reason decides to shove for $550. Action folds around to the straddler who is going to look at his cards, but still has not. Player A thinks everyone has folded, and mucks his hand. The hand is not retrievable as the dealer places it in the muck. The straddler gets mad and claims he would have called with KJos. Floor gets called to make a decision regarding the hand - what's the right call here?
With the logic of "the hand is dead, straddler has the right to call and win the entire stack of UTG +1", then you open up for chip dumping. The correct call is to declare the hand a misdeal. Keep blinds where they were at, shuffle up and deal. Also, warn the UTG player to pay better attention, and tell the straddler to calm down as he is not entitled to anything here. You cannot play an all-in versus a dead hand. This is by far the best decision for the game, and shuts down chip dumping possibilities.
We are all humans and make mistakes. Don't be that guy who greedily feels entitled to anything here. In all honesty, do you really want to win like this anyways?