When "an opponent" has to do something in a multiplayer game, who does it?
The player who controls the spell chooses the opponent.

There could be some political issues here, but no more so than if you had a friend in a 4-player game who refused to attack you because you were friends. He's still your opponent in the game, so he should be making the decisions that will give him the best chance of winning; not trying to help you.

From the comprehensive rules:


  601.3. Some spells specify that one of their controller’s opponents does something the controller would
  normally do while it’s being cast, such as choose a mode or choose targets. In these cases, the
  opponent does so when the spell’s controller normally would do so.
  
  
    601.3a If there is more than one opponent who could make such a choice, the spell’s controller
    decides which of those opponents will make the choice.