[abby99]

From Robert's 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). Verbally stating one’s hand during the play may be penalized.

You made the correct ruling, assuming of course that you follow RROP. The player should also be advised that exposing one's hand is not permitted and that doing so again could result in a penalty.

Because it was obvious that SB knew he shouldn't have turned over his hand, I wouldn't make a big deal out of it. Now way would I declare the hand dead.