That didn't work.
I'll copy below a fix that works, from our IT guy who wishes to remain nameless. He did some incredible sleuthing.
I learned a bunch about Time Machine from this exercise.
From your Time Machine Disk:
Go to the appropriate backup in the Finder.
Note that this doesn’t work using the standard Time Machine ‘Restore’ from the menubar, since invisible files remain invisible from there.
Show invisible files by typing: cmd-shift-. (period)
Navigate to the path below, where ~ is your Username:
~/Library/Application Support/com.apple.sharedfilelist/com.apple.LSSharedFileList.FavoriteItems.sfl2
Copy this file.
Open Terminal and type:
killall sharedfilelistd
Paste the replacement file from the Time Machine backup into
~/Library/Application Support/com.apple.sharedfilelist/
Use Apple Symbol/Force Quit on “Finder” which will Relaunch it.
Finder Favorites should be restored.
I'm keeping a copy of a functional version of com.apple.LSSharedFileList.FavoriteItems.sfl2
on my desktop in case this happens again.
Good luck!