Ensure that each user has granted access to their Google Drive account while logged into their Google user account. If you have configured an org wide google authorization, check that this Google user is the client’s Google user. If needed, have them create a free Google user so they can use that to manage templates.
Verify that all Google Doc templates in the document actions and all Google Drive folders are owned by the client’s Google user. You can change the owner of Google Docs and Google Drive folders to your clients Google user. Check that the Google Doc templates and Google Drive folders are shared properly. If you make the client the owner, then this should not be an issue. You can change the client to the owner and then share the Google Doc templates and Google Drive folders with your own Google user set to edit access if you’d like to be able to provide support. Best practice we suggest is to transition all ownership over to the client.
Generally, the person who needs to edit the Google Doc template should be the owner or at least have edit access. Then that template needs to be able to be viewed by anyone with the link. Anyone generating any new Google Docs must have edit access to the google folder specified in the related Document Action.
Test logged in as the client Salesforce user and also logged into their Google user account. Alternatively, you can ask them to test it on their own or with you watching to make sure everything works properly.