If someone else registered the viewer software from the same computer that is now being used by a different user, then the new user will be able to view the protected files that the previous user could. This is true even if the new user registers with a new license file on that computer, because multiple licenses can be used on the same computer, and the user will be able to view protected files granted for both licenses.
You can test this by either protecting information as a standalone document/product or to a new publication which no one has access to. You will find in both cases the user will not be able to view that file.
Or alternatively, ask them to remove their keystore – from the Windows Start menu >> All Programs >> LockLizard >> “product name” >> Viewer >> Remove keystore (where “product name” is the LockLizard product you are using) and then see if they can still view the file.