In this case, because the action was on the river, could Player 3 act and then it is determined how to proceed from there. Player 1 has already bet and been called by Player 2. If 3 raises, obviously Player 2 needs to be shown Player 1's hand. But if 3 calls or folds, Player 2 is going to see the cards at the showdown anyway.
I understand the rule, but in this case since there is one player to act and he has seen the exposed cards, I think you could wait to expose 1's hand to Player 2. Sort of a "six of one, half dozen of the other" situation.
I would say that the cards need to be made available to Player 2 once they had been exposed. If Player 2 is afraid of giving off a tell or whatever then he is free to not look at the cards if he wants but I see no reason for the cards not to be shown immediately after the infraction.