I believe the manager explained his/her problem when she wrote: "My relationship with this employee has been purely professional and I cannot leverage any personal loyalty or respect in any discussion."
WAKE UP!
Professional does not mean impersonal. It means doing the job competently and with due dillegence. And if the job is managing people, then you better damn well know how to inspire loyalty and respect.
There is no good solution for the manager in question except to get used to dealing with all the myriad problems associated with emloyee turnover.