All In Calls and Flipping Cards

[jnh24]

I played in a home tourney last week and there was a situation where I was the lone dissenter of the group.

Heads up, after the flop, a player goes all-in. THe opponent then flips his own cards over, to gauge the reaction of the all-in player.

I thought that exposed cards were declared dead, and in such a situation the hand was declared dead. Everyone else swore that this move was legal, and one even said he had seen it occur on TV.

Ruling?