I highly recommend the following:
Smith & Wesson 9mm
Colt Mustang 380
Taurus UltraLight 38
I've told this story here before on similar threads, but I'll tell it again. We were in the process of moving and my wife was in another city setting up things on that end. She got into a dispute with a man at work and he started stalking her. I went to the gun store and bought her a KelTek P-11 9mm to carry in her car. It's simple, lightweight, and packs enough punch to discourage a bad guy. She fired it once and refused to shoot it again. It didn't fit her hand, it was too loud, and the recoil hurt her hand. She ended up taking my Colt Diamondback in 38 special. It's a medium frame revolver that doesn't have the sharp recoil of a lighter gun.
The lesson is that you'll waste your money if you try to pick out a weapon for your wife. Let her try out several guns and pick the one that suits her.