A situation at the poker table

[Foo King]

Three people left in the pot, low limit game. Player 1 at one end, player 3 at the opposite end and player 2 in the middle across from the dealer.

Play on the river: player 2 bets, player 3 calls, player 1 calls and raises all in. Player 2 calls the all in raise, forgetting that there is another player left in the hand, player 3, and player 2 lifts his cards and starts to turn them over, inadvertently exposing the faces of the cards to player 1 but the backs of the cards are still towards player 3.

The dealer says, "Wait, wait, wait. There is sill another player in the pot."

Player 2 says,"I'm sorry. I forgot he was still in this pot. Go ahead. Your play."

Player 3 says, "Since player 1 saw your cards I want to see them too. If one player sees them then everybody sees them."

Dealer says, "Yeah, that's right. You have to show him your cards too."

At this point player 4 speaks up, "No. That's not correct. Player 1 recieved no advantage because he is all in. Player 3 should not be entitled to the advantage of seeing player 2's cards."

Discussion ensued.

Me? I was just a bemused spectator, player 5. The rest of the table and I watched and listened with interest and a touch of amusement.

What is the resolution in a situation such as this? Opinions?