I'm not a big fan of chemical weapons to much to go wrong. This leaves guns and knives. Some elderly individuals have trouble with guns. Recoil, arthritis and carrying it. These may or may not be an issue with your friend. The knife is arguably faster to deploy easier to carry and conceal and again arguably more effective. Of the two the gun will take less time to learn and will provide protection from two and four legged encounters.
As Tolly pointed out using a knife is very nasty. Acquiring the mind set and the technical takes time.

If she can legally, in reality a small caliber revolver like a .38 or even a .22 would be her best bet. And add to that some good training.