ah, so in the end it was simple, just unfriend them but do not revoke library access.
I did not realise they do not have to be a ‘Plex Friend’ to have access to my libraries.
(But then it has been a long time since I added anybody, as the onboarding process is now way too complicated for new users)
If I need or want to see what they have watched from My server I can still find this in the dashboard.
