We have this same conversation every time someone asks, "My wife/girlfriend/mother/daughter wants a gun, what should I get her." And every time I give the same question: it's up to her.
The most important factor in a woman's effectiveness with a firearm is
her confidence to use it. If it's a gun she's comfortable operating, that she can take to the range and put shots on target, that
she enjoys shooting, than it's the right gun for her. Doesn't matter if it's an AR or a .22 revolver. Only hits count.
So take her to a gun shop, or better yet, a public range with a rental arsenal, start putting guns in her hands, and see what she tells you.
A semi auto handgun might be a great gun, but does she have enough grip strength to reliably rack the slide? I had to cross off half the list of potential guns for my wife because she couldn't rack the slide.
A long arm may be a more effective weapon, but they tend to be very heavy and require two hands (making moving through a house, opening & closing doors, holding a flash light or making a phone call very difficult).
A full sized revolver could be a good solution, if she likes the way it feels in her hands and isn't afraid of the recoil.
But the only way you're going to figure any of that out is by putting the gun in her hands and letter her decide for herself.
And you never know exactly what she's going to like. I had a tiny ex girlfriend (she was 4'11", weighed something around 100lbs) and her handgun of choice turned out to be a big, hulking Beretta 92FS. Everyone here would scream at me for buying her that gun, telling me it's too big for her small hands, but she loved it (and she was very effective shooting it). Go figure.
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I go with Len, a 4" K frame is ideal, or a Ruger sp101 4". hot 38's or 357mag
something she can handle safely, look at the cylinder and know it's unloaded.
Buffalo Bore makes low flash/low recoil .38 special rounds that sound ideal for this situation.