Ordinarily a hand is dead if it hits the muck. If it can be clearly identified then there are situations where it can be resurrected though. From Robert's Rules (http://www.homepokertourney.com/roberts-rules-of-poker.htm#SECTION_3_-_GENERAL_POKER_RULES):
2. Cards thrown into the muck may be ruled dead. However, a hand that is clearly identifiable may be retrieved and ruled live at management’s 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.
In this case, the player has been given incorrect information (by the dealer, no less) so I think there's a solid case to be made for resurrecting the hand from the muck and awarding the pot to the rightful player - assuming the cards can be identified and the correct number of chips can be returned to the pot. If it's a friendly game I'd even be inclined to ship the pot (or as close an approximation as could be made) to the player if everyone at the table was in agreement that they definitely showed the winning hand.
So the suggestion that once cards hit the muck they are always
dead is completely wrong. They're dead most
of the time, but not all of the time. You can't just read a single line in Robert's Rules, you need to read the exceptions as well.