If we're actually at showdown (ie: all the betting is complete) then it sounds like this is a no-brainer: dealer just turns the cards over and if either of them has already gone into the muck, every effort should be made to retrieve them from the muck because it sounds like an honest mistake: player was just showing his hand and one of them accidentally flipped back over.
Here's the relevant rule from Robert's Rules of Poker Section 3, Dead Hands
2. Cards thrown into the muck may be ruled dead. However, a hand that is clearly identifiable may be retrieved and ruled live at managementís discretion if doing so is in the best interest of the game. An extra effort should be made to rule a hand retrievable if it was folded as a result of incorrect information given to the player.