Show one show all, applies in this circumstance, however I am unlikely to honour a players request to see a mucked hand, poker is an individual sport and players have the right to muck a hand as they see fit. Applying the show one show all also requires that if a hand cannot be displayed (because it is in the muck) that the player AT LEAST in good faith disclose the contents of ones hand, again even when a player (heads up) has bet the river, been called and then immediately mucked when showdown was reached, the hand is over, and the other player gets the pot without showing also as he has the last live hand at the table, again the player who called the river bet can ask to see the mucked the hand - but it's unlikely I'm letting them see their opponents hand. That's the way I see it under current TDA rules.
Regards
Stuart