Before doing anything he should have made a declaration as to what his actions were going to be. At a Casino he would have been reprimanded, although I'm not exactly sure as to wether he would have been forced to fold or not.

That's the one thing about friendly home games. People think they can do anything. If it were my home game he would have been forced to fold. And told no cards are to be shown until the pot has been awarded.

You can also check out Home Poker Tourney for The Official Rules of Tournament Poker.