It is the pharmacy's responsibility -- and the responsibility of any human practitioners that may be treating this client -- to monitor prescription activity via the PMP Registry and take appropriate action before dispensing or prescribing.  Practitioners other than veterinarians have access to education/training with respect to individuals that appear to be "doctor shopping" and may be abusing substances. 

If you believe the medication is being diverted and abused, you've already done the right thing by refusing to write prescriptions.  Our legal counsel advises that you continue to do so in order to protect your own liability.  (This could damage your established relationship with the client; however, it may ultimately be the most beneficial course of action for you, the client and her companion animal.)