Rules question

[BrianSwa]

was at Taj last night and was wondering something, dude bet into my buddie on the river and my buddie called and the dude showed the winning hand and my buddie tried to muck his hand. The other guy requested to show his hand and the dealer showed my buddies hand. We got into a discussion because some of us thought the person could muck without showing but the dealer and player said if requested he had to show hand. Is this true if it goes to showdown the other player HAS to show.