The practice guidelines state that a veterinarian should only prescribe treatment when a valid VCPR has been established (see http://www.op.nysed.gov/prof/vetmed/vetpg5.htm, Section 5.12).  

A VCPR exists when all of the following are satisfied:

  1. The veterinarian has assumed the responsibility for making medical judgments regarding the health of the patient with the assent of the owner of the animal or their duly authorized agent.
  2. The veterinarian has sufficient knowledge of the patient to initiate at least a general or preliminary diagnosis of the medical condition of the patient. This means that the veterinarian is personally acquainted with the keeping and care of the patient by virtue of:
    1. a timely examination of the patient by the veterinarian, or
    2. medically appropriate and timely visits by the veterinarian to the operation where the patient is managed, or
    3. medically appropriate and timely visits by the patient to the veterinary facility where the veterinarian is working.
  3. The veterinarian is readily available for follow-up evaluation and oversight of treatment and outcomes, or has arranged for appropriate continuing care and treatment.
  4. Patient records are maintained.

Your patient records need to reflect the new owners and contain their consent to treatment, physical exam results should appear in the patient record, etc.  (It is also recommended that a VCPR exist prior to dispensing non-prescription -- or "prescription" but unregulated by the government -- flea products.)