Let's say there's a senior citizen player at the table who is slow to act but seems competent and aware of the rules.
The board is 7
8
9
3
2
The senior citizen is all in with a young man on the flop
At showdown, the young man tables his JTxx (no flush) for the nut straight.
The old man only tables his J
T
while holding back his other 2 cards.
The dealer starts chopping the pot.
The house rule is you must show all 4 cards to win the pot.
Is it against the rules or unethical for another player to tell the old man he has to show his other 2 cards? There's a possibility he will accidentally drop and muck the other two cards, making him ineligible to win.
Does the "one player per hand" rule apply here?