Worksets or borrowed objects are owned by a Revit Username.
The only way to relinquish Worksets or borrowed objects is from within a Revit session that is using that username.
This could be done by starting a Revit session with an install using that username.
If you can't do this (because you don't know their windows password) you can 'fake' that user.
Save to central or Close the Revit file you are in (make sure YOU relinquish everything).
From R top left corner, Options, General tab.
In Username box type in the owners name EXACTLY.
OK out of dialog box.
You should now be able to relinquish all their worksets and borrowed objects.
Make sure you change Username back to your name.