I don't know what kind of things she wants to transfer, but Mac file formats may not be readable by a Windows PC. Mac is much friendlier to Windows formats than Windows is to Mac.
If she has jpegs or MP3's or other universally readable files she's wanting to recover, you may want to try installing the iBook's hard drive in a suitable external enclosure such as
this one (assuming the HP has FireWire). If the HP doesn't have FireWire, you could try retrieving the files to your PowerBook and then transfer the files to her HP.
Instructions for removing the hard drive can be found at both
iFixit and
PowerBook Tech.
Good luck.