First off,. talk to the police. They should be able to enroll her in some kind of witness protection program and either assign a officer to her side 24/7 or move her to a safer location.
I'd suggest buying something a little more powerful than pepper spray(I'm a intern at the local police department and I've seen people not be affected by police strength pepper spray).
Buy a Air tazer, it's the same thing the cops use. It sends two probes that are like fish hooks into the body of the perp and with a pull of the trigger sends electricity through the perps body. Highly effective at stopping someone but still non lethal.