Situation in NLHE tournament: One player was betting on heads up river and got called. He mucked his hand and said "The pot is yours." Does the second player have to show his hand to claim the pot?
No the second player does not have to show his hand. If a muck is allowed than he does not have to show.
This should again be written in the "house rules". In some Casinos every hand has to be shown at a paid river. Last action shows first. Calling is not an action. So the guy who bets / raises lasts should show first.
But if he is allowed to muck his hand, why shouldn't apply the same rule for you after taking the pot? ;)
In fact this seems to be a very variable rule. Asking in a casino nearby about the house rule regarding this issue they said that they changed the ruling in february. Before that, you had to show your hand on showdown even if all other players had mucked. After that, they changed it that you don't have to show.
The 2015 Poker-TDA-Rules also state that you don't have to show:
16: Non All-In Showdowns
B: A non all-in showdown is uncontested if all but one player mucks face down without tabling. The last player with live cards wins and he is not required to show his cards.
Almost all casinos have a rule that says you need to show two cards to win the pot.
They also all have a rule that says the last live hand is the only hand that has claim to a pot.
So the second player has claim to the pot rather or not he shows the hand.
But also it is clear that the rule says at showdown he is required to show the hand.
If one gets technical the question becomes what rule trumps the other or even does one rule trump the other. But the debate about that would be wholly academic.
Based on decades of experience in the industry, what happens the dealer simply pushes the pot to the last man standing. If anybody raises objection saying something like "hey he has to show his cards to win the pot", the player must show his cards. As the dealer you protect your muck from his discard, ask him to show his cards, if the player refuses you call the floor person, who if he is not confused will tell the player to show his cards, or reach over and show the players cards. (People often confuse this situation with the rules about seeing a players hand, they are separate. Floor people are the ones that most often get confused here)
Customarily it is not the dealers duty to insist that the last man standing in this situation is to show their cards.
I am not saying that this is right or wrong, just saying that customarily this is what it is. I personally think that they should always be obligated to show the hand, because when they don't it slightly diminishes the security of the game.