it depends on a few things.
first if it is the button that has only one card (because the dealer forgot to give him his second card then you simply give him the top card from the deck (up until his turn to act)
any other position will be declared a misdeal if is noticed before a third action happens (call, raise or fold).
once a third person has acted the players hand will be declared dead. The more than 2 actions rule is to prevent players from angling.