Thank you, Pajamaman.
Maybe I shouldn't have mentioned the Kobo. I'm fine with organising things on the ereader; I use the tags in calibre to sort the books into Kobo's "collections".
My real questions are ...
I want to be able to add metadata etc etc on MY computer and then update HIS library with that new info.
One way seems to be to copy the updated library folder from my PC to his and then use "switch/create library" and make a new library each time - as shown here:
https://www.youtube.com/watch?v=CemyyMTbNMM
I'm happy doing that because it's easy.
But
while doing this metadata updating, I've been making rules in "Preferences>Metadata download>Create rules to filter/transform tags".
How to get those rules from my PC to his without redoing them?
Also, at some point I'm going to want my own library separate from his.
How can I then continue to do work on 'his' books on my PC and then update only those books on his PC?