Much good advice so far. I would suggest she first take a class in hand-to-hand self-defense. While being armed and ready is obviously #1, It's possible to be caught off-guard by a stealthy intruder.
For a 90-something pound person, eye-gouging, throat-chopping, and nut-kicking are a few things that can give you precious time when faced with someone 100 pounds heavier.
Finding a gun that suits her should come a little later.