Also, if it is indeed a 12" model, it will not have a CardBus/PC Card slot, so AirPort Extreme or USB are the only two options.
The 1.33 GHz and 1.5 GHz 12" PowerBook G4s had the AirPort Extreme card included, so if it is one of those two speeds, he should be set. The 867 MHz and 1 GB 12" models had it as optional, though.
And if you have the screen size wrong, and it's really a 15" model, then there is the possibility that he has a Titanium PowerBook (the one with the ports on the back instead of the sides,) which takes an original AirPort Card. The newer Aluminum 15" models (ports on the sides,) takes the AirPort Extreme Card. Also, all 15" (and 17") PowerBook G4s have a CardBus/PC Card slot, so you could use a (possibly cheaper) CardBus card, which would have the added benefit of getting better reception than an internal card on the Titanium models.