In fact, you don't any longer need to be an Exec Club member to check in at -24 hours. Anyone can do it. So all you'd need is his booking reference. If you're confident that he's going to travel as booked, check him in online, pick him a good seat and (as wub says) don't print the boarding pass if you're given the option. He can then just pick up the boarding pass at his departure airport.
There's no technical reason why you shouldn't do this - I do it quite often for people who are overseas and without Internet access at the time that check-in opens.