In a multiplayer game, if a player is controlling another player's turn, may they reveal that player's hand?
The controlling player cannot reveal the controlled player's hand. Rule 714.5b says


  The controller of another player can’t make choices or decisions for that player that aren’t called for by the rules or by any objects. The controller also can’t make any choices or decisions for the player that would be called for by the tournament rules.


Revealing your own hand in accordance with tournament rule 3.12 is a decision that would be called for by the tournament rules, so the controlling player is not allowed to make that decision. By that same logic, the controlled player would be allowed to make that decision.



I don't have a solid ruling for this part, but my interpretation is that the controlling player can verbally name the cards in the controlled player's hand. Revealing your hand is a game action that involves game objects, so it is covered by the game rules. However, there are very few rules about players' verbal communication, and there are none that govern statements that players make about hidden information. In other words, there is no rule against naming those cards, and there is no rule against lying about them.