Well, I will ALWAYS recommend a GOOD "wheel gun" over a semi-automatic when it comes to self defense. There are a LOT of reasons for this, but the main ones are as follows: no safety to "deactivate" the gun at the wrong time so it is quicker to bring into action when fractions of seconds count, MUCH less likely to "jam" than even the most reliable autoloader in the world, if the first round happens to have a bad primer you get a second round on a second trigger pull immediately without having to work a slide, you can beat someone with it if necessary and it'll still fire usually flawelessly, MUCH simplier to learn to use and operate, much safer for a DA since you can open up the cylinder and make SURE it is empty while people DO forget about the round in the chamber when they drop a mag OR empty the chamber and THE drop the mag leaving the gun in fact loaded, etc.
As far as WHAT calber of revolver, I believe that the .32S&W Long is the BARe minimum to use, but it is still too light, good ammo is hard to find, and the grips are usually pretty bad. BUT, anything larger than that should work. Recoil shy hands don't want anything too large, either, so I'd definitely stay smaller than the .357 Magnum.
My suggestion is the time proven .38 Special. Ammo is VERY available, cheap, recoil is light, bullet type is VERY available, you can purchase from light loads (practice) to +P+ loads (self defense), etc. If she plans to carry the gun, I'd get a semi-light .38Special 2" snubbie. (Not too light, though, or recoil will be bad.) If she doesn't want to carry, and will only keep it in the car or the house, then I recommend getting a .357Mag and shooting .38 Special rounds out of it. That'll give you a VERY well built gun that you COULD shoot hotter rounds out of if YOU decided to use it, too. I'd recommend a 4" barrel for a non-cary gun since it is a good compromise between the compact 2" and the more accurate 6+".
Will all of my options, my "owner" carries a .38 Special ultralight snubbie and I carry a .357 Magnum Snubbie (but I use .38 Specials in it when I'm in the car out of concern for my ears if I ever have to shoot it in a confined space....lol).
Remember: Shoot a lot + Carry a little = heavy gun. Shoot a little + Carry a lot = light gun. OR, if you are a "gun nut" like many of us, shot a lot + carry a lot = heavy guns + light guns and shoot and carry both a lot....lol.
KJ