I waited for him to finish, and then asked him politely how he proposed that I do a backup of files that I'm not allowed to have any access to?
I mean, set up Borg to back up his machine, in theory, but you'd have to set up backups first, and then blindly trust Borg that nothing's going wrong. And of course Borg doesn't do Windows, so you'd need his machine to run an SMB server