Couldn't you add the FF number just before boarding? As long as a gate agent scanning the boarding pass is the event that triggers crediting miles to a FF account, as long as the FF number has been added to the reservation just before boarding, I'd think that it would be fine. I would assume that a gate agent would balk at adding a FF number, but if you could even let your relative board first and then wait and have a gate agent add your FF number (or even call US Airways and have it done by phone) just before you board, then I'd think that would work to ensure that you get credit for the flight.