Thanks for posting your question here in the Community, ToddAndMargo.

I want to make sure your customer is able to backup their .qbw file in QuickBooks Desktop without any hiccups.

Here are a few ways to determine which workstation is creating the file lock within the account:

View Logged-in Users:

• On any workstation where QuickBooks can be opened, navigate to the Company menu, then select Users, and finally View Users.
• This will display a list of users currently logged into the company file.

Identify the User/Workstation:

• If the error message explicitly mentions a specific user, check if that user is still listed as logged in within the View Users window.
• If the error persists and a user is still displayed as logged in, that workstation might be the source of the file lock.

Troubleshoot Hosting Settings:

• On all workstations (except the server/host): Open QuickBooks, go to File > Utilities. If Stop Hosting Multi-User Access is present, select it and confirm. This ensures only the designated host is providing multi-user access. 
• On the server/host: Ensure Host Multi-User Access is enabled under File > Utilities.

The following article provides additional info about fixing locked file errors in QuickBooks Desktop. Please feel free to reach back out if you have any other questions. I'll be here to lend a hand.