any user with administrative priviliges can select the object in DMS Desktop and choose "Check-In".
You will receive a warning from DMS that the object is checked-out by USERX, but you can click "Yes" to release the lock. Please note that any work done by USERX will be lost unless you grab their data from their Sandbox.
Within LFM, just click refresh and the lock will be released.
I hope that helps.