From your wife Mac update all the settings in system preferences ( you have to fill up every credentials and verify it ) see the article https://support.apple.com/en-in/guide/mac-help/mchl3f671010/10.14/mac/10.14
You have a primary Mac A and other devices like iPhone / iPads / your wife Mac B are signed in with same Apple ID and password on the same network say example1@icloud.com , the iCloud Drive is set up https://support.apple.com/en-us/HT204025
The desktops and documents are also turned as on see the article https://support.apple.com/en-in/guide/mac-help/mchle5a61431/10.14/mac/10.14
The documents created in both Mac will mirror see the section - Access your Desktop and Documents files on another device
in the article https://support.apple.com/en-us/HT206985
She has turned on desktops and documents from Mac so they will always sync with iCloud and can be viewed at www.icloud.com
She can only drag and drop documents to some other location from Mac see the article https://support.apple.com/en-in/guide/mac-help/mchl1a02d711/mac
Move items to your Mac and remove them from iCloud Drive
- On your Mac, drag items from iCloud drive to another folder in the Finder sidebar or on the desktop, such as Downloads, Movies, Music and Pictures.
- The items are copied to your Mac and removed from iCloud Drive on all your devices set up for iCloud. If you change your mind, you can drag items back to iCloud Drive.
Still you have set universal iCloud account for all devices the documents are still saved on iCloud.com
Its her wish to sign out the the Apple ID account from Mac and get separated from syncing devices , first delete her personal documents from iCloud Drive they will be deleted from iCloud.com as well and then simply sign out https://support.apple.com/en-in/HT208242
For your Mac you as organiser see this https://support.apple.com/guide/mac-help/delete-shared-folders-or-files-mchl46e62ad0/mac