Is that the only administrator account? There has to be at least one admin-level account. He should create the new account, then delete the old one.
Personally, when selling a machine, I'd prefer to put the system environment in an as-new state: do an Erase and Install from the Tiger DVD, install iLife and any other apps, use Software Update to get everything up to date, etc. Finally, there's a .plist(?) file that you can edit to make the 'Welcome' screen/vid reappear on the next boot, to make it look like the first time booting the install. I saw how to do this on the MacNN forums a while back but can't find it now. You might want to bounce over to their OS X forum:
http://forums.macnn.com/forumdisplay...aysprune=&f=90