Criteria:
Be small enough and light enough to carry.
Must be streamlined enough to NOT hang up if getting out of purse pocket in a panic.
Be simple enough to “point and shoot”; no slide, no safety, no hammer to cock.
Have enough stopping power to actually deter an attacker.
Any gun left at home is ‘no gun at all”. Women and men will often panic if they need to quickly pull out their gun in defense. A gun hammer will almost always hand up on a pocket or pouch, thus a hammerless type gun is preferred. No hammer, no need to cock – it’s always ready to fire.
My preference for the lady would be something like a S & W Airweight .38 Special. Not absurdly expensive, small and light, cheap ammo for practice, and simple to use in a panic. It will be much more likely to be on hand when needed.
Now – that would not be MY preference for myself. I like a big caliber gun; a cannon if I could carry one. But The Lady, consider HER needs and capabilities, not your own.