I'm not going to recommend a firearm because I don't know the ladies familiarity or lack thereof on firearms. I will tell you that my wife does very well with a 12ga shooting reduced recoil 00 buckshot.
To the people recommending #4's or whatever other kind of birdshot STOP. Very bad advice, and you may get someone killed. It has been proven time and time again that birdshot leaves a very nasty shallow wound. It is not sufficient for home defense. It never has been, and never will be.
Anything that will penetrate far enough to vital organs will penetrate thru walls in your home. Birdshot is for birds, not stopping a person intent on doing you bodily harm.