What should happen if a player intentionally shows his hole cards to another player who is not in the current hand before the hand is complete
There is no penalty for this. It is not against the rules to show a hand to other players or to someone on the rail or sitting behind you.
If the hand is head up between two players other players can ask to see the hand. In the case where there are three or more players and the hand does not make it to showdown, the hand is mucked. No show.
If the player that is shown the hand offers advice on the hand, the players are in violation of the one player per hand rule. Rarely is their any penalty for this but often a warning by the dealer or a floor person is issued.
It is against the rules to show your cards to other players whom are still in the hand. In this case the cards are turned up by the dealer, and the player is told not to show his cards. Repeated offences will result in action from a floor person. People can be asked to leave the game for repeated offense. In poker tournaments the rules are more formal and restrictive. Offense can result in penalties up to including being disqualified. It is very tacky, and often an angle to show your hole cards to other players still active in a hand.
But as far as showing to someone not active in the hand, besides slowing down the game a bit, it is no big deal.
See Can a player expose his cards to another player in a no limit texas holdem tournament? for a slightly different situation.