I know he doesn't need the file association for regedit to make regedit work.

But my worry is that the thing that's broken is the association for .SCF files, and that's why the microsoft KB fix isn't working for him.

If the association for SCF files is broken, maybe other file associations are broken on the machine as well, pointing to an even larger problem. That's why I think the TweakUI "fix default file associations" trick is worth trying.