Is there a particular reason to leave MacOS intact? After removing all
the user data you might as well have a fresh copy of the OS, so I'm not
sure why you wouldn't zero the entire disk and then do an OS reinstall.
Owner of the thing does not have a set of installation disks. so, if
erased, it would be sold or given away unbootable. I suppose someone
interested enough in vintage Macs to acquire it would already be equipped
with installation disks, but, it would be nice to send it on demonstrably
working. They said they tried Googling a solution and didn't come up with
anything, I took their word on that. Saving a bunch of very large files
seems a reasonable way to do it, I'll suggest that.
thanks
brian