Unfortunately, you are going to need an OS 9.2 disk. OS 9 needs to be present to install 10. If he has a password on the volume 10 can't install, which sounds like your problem.

You'll most likely need to boot off a 9 disk and reformat from there.

You can get an OS 9.2 license pretty cheap from these guys:

www.smalldog.com

www.macresq.com

Another question, what happened to the disks that came with the machine?

I just hope he didn't put a firmware password on it...