Eddy: To add to Uniden's reply:
1: In your example there was no betting on the river and it's not an all-in showdown. Hence no cards must be shown if all but one hand mucks face down.
2: If there was betting on the river, any caller of the last aggressor has a right to see "the hand he paid to see" on request. This was voted in overwhelmingly at the 2013 Summit, it just wasn't then written into the rules in 2013 because it could leave the impression that's the only condition for which a request can be made. Per TDA rules, the requesting caller must retain or have tabled his hand. Note this right does not extend to one caller asking to see the hand of another caller.
3: If in your example there was a player all-in prior to the river, all hands would have to be shown.