If it is in the home where the gun doesn't need to be portable, I would get her 20 gauge 870 pump with an 18.5" barrel and load it with a couple rounds of bird shot and then buckshot for the rest. They sell "tactical" models that hold 7 rounds. Usually it only takes chambering a round for the bad guy to take off.
Pump Action Shotgun - Model 870 Express Tactical - Remington Shotguns
George