Sounds like it is a 'dead hd' given the circumstances you said. You could always try and take the drive out, and put it in an external enclosure to see if it is recognised on another machine.

Aside from whatever identification strings and machine-specific features that may or may not be enabled in OF (i.e, video spanning), the iBook G4, PowerBook G4 Al, late-model eMac, and Mac Mini are virtually the same, excepting A/V subsystems; they've all got the same Intrepid system controller and AirPort/USB 2.0/modem components, PowerPC 74xx-series processor, etc. So, software that doesn't check the hardware ID strings in OF will have little reason to run on one but not the other.

I've used restore discs from 800MHz iBooks to install OS 9.2.2 and OS X 10.2 on machines with compatible hardware (i.e., those based on the Grackle/UniNorth/Pangea system controllers), and have had no problems, other than the iBook-specific documentation, and the iBook registration software that starts on the first boot of 9.2.2.

So as long as your client was using software from an iBook G4, there shouldn't have been a problem. I'd guess that the hard drive died or became corrupted to some degree. Of course, it could just be a case of loose connectors or something, as well. Grab a putty knife and have a look, maybe pull the drive and try to mount it in a different machine or something while you're at it.