If a player announces to the table after the river with one player left that if the other player has a pair you win and then places the cards face down on table? Is the hand "dead" for the player who faces his cards down or can the dealer or player, after an outside observant informs the player that faced his cards down, he has the winning hand? The player or dealer then re-opens the hand face up?
Yes, the hand is live. But if it was not properly tabled face up, anyone who saw the hand should keep their mouths shut, and the dealer should muck the hand if it is unprotected. If the hand is protected, then the dealer and other players in the hand must ignore anything said verbally and encourage the player to either properly table the hand or properly muck it, and must not take further action until he does. The dealer should also try to protect the hands of the other players. In extreme cases, the floorman may intervene to remedy a deliberate angle shot.