I suppose if the scenario allows for it; other than that, I think it should be assumed that the players are above betrayal of their country. If it was just one player who decided to do it on spur of the moment, I'd say that's a little ridiculous. It's not any kind of malevolent offense, though, so I'd say warn the player, and if it happens again, ban him from a game. Don't be too hard on him. Just know that's kinda lame/stupid.