What happens in texas hold em when a guy flips over his hole cards before the river? Is the hand dead then?

What happen at a poker home game, was a new guy who was playing mistakenly thought the turn was the last card and showed another guy he had a straight. There was lots of money in the pot and we did not know what to do. So, I said it was a dead hand and everyone gets there money back of course to the dismay of that guy with the straight. What is the actual rule?