I'm so sorry this happened to your patient. I don't understand why any Dr would do this to a patient knowing what could happen with having seizures. Did she say why her Dr did this? Can your office prescribe her medication until she can can find another primary Dr?
You're right -- it can cause seizures with a history of high-dose long-term use.
It's not illegal, but can be unethical, depending on the length and dose of use. Patients who are dependent on benzos and cut off their meds usually end up in the Emergency Department.
Writing the medical society can cause trouble for the prescriber but won't immediately help the patient. If the patient is showing signs of withdrawal, then he/she should seek treatment, whether with another doctor or with the ED.