The TDA rules cover this, see rule 8:
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8 At Your Seat A player must be at his or her seat by the time all players have been dealt complete initial hands in order to have a live hand. Players must be at their seat to call time.
[/ QUOTE ]
He's not at his seat, his hand is dead.
Obviously in a home game we give folks slack on this one, usually the common practice being as long as you are at your chair when its time for your action, or on your way, your hand isn't killed.
Here, the guy isn't at his seat his hand is dead, he doesn't ever get to have a live hand, as in the next example: suppose the absent player in the BB wasn't all in, but had chips left, but the pot is limped around, his hand doesn't get to stick around for the flop, its dead, even though he could have checked it if he were there.
--Zetack
Edit to provide link to TDA rules:
TDA Rules