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In our home game you have to show your cards no matter what during an all-in and if you much the dealer will generally flip them over. We had an incident like that once and this is how we avoid it now.

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The all in was called on the river, so the hand was over, it's not unusual for an all in to be called on the river and the winner to flip over his/her cards and the other player muck their cards.

The Robert's rule is pretty complicated regarding if a non-winning player asked for the hole cards that they're deemed dead, well, not complicated, but if you tried to use that rule at the table when the donkey realized he had the flush he would have wanted the pot too.

Tell the player to just always flip over his cards and shrug his shoulders and if the other play in the hands curses, that's his cue to scoop up the pot.