http://www.homepokertourney.com/showcards.htm

Roberts Rules of Poker. Section 15 - Tournaments. 21. Showing cards from a live hand during the action injures the rights of other players still competing in an event, who wish to see contestants eliminated. A player may not show any cards during a deal (unless the event has only two remaining players). If a player deliberately shows a card, the player may be penalized (but his hand will not be ruled dead).


http://www.pokertda.com/rules4_inter...sults_page.asp

35. Expose. A player who exposes his cards during the play may incur a penalty, but will not have his hand killed.


A penalty is typically a forced time out of the game, i.e., the player may not play another hand for 10 minutes.

Certainly, it's very poor etiquette and shouldn't be allowed to happen again.