Have the user tell you the number which appears in their APPX Client title bar (they've presumably done this already, but in case they haven't, now's the time to ask). That is the NT PID (process ID) of the SERVER process to which this client is connected. Once you have the PID, but not before, instruct the user to bring up their Windows Task List, and select the APPX process, then click on "End Task". It may pop another box up, saying that the process is not responding. Instruct the user to click "End Task" on that box as well.

Wait a minute or two for an orderly shutdown of the server-side process connected to the client. APPX should detect that the client has disappeared, and end the server process.

If after waiting several minutes, you still see the server-side process ID in your NT server's task list, you will need to manually kill the server-side process.