Depends. Revolvers have a much shallower learning curve - for the reasons mentioned.
If it's going to get carried about, just teach her "five beans in a wheel" - the chamber under the hammer, when stored, should be empty. This helps to prevent NDs. When the hammer is actually cocked (by the thumb or the trigger,) the cylinder will advance one, and there will be a round under it when it's done.
There is no safety. Don't want the gun to fire? Keep your finger off the trigger.
Revolvers don't jam, stovepipe, fail to feet, fail to extract, or fail to eject. No IA drills to teach.
The grip on a revolver tends to be smaller, since there's no magazine.
I'd suggest getting a revolver chambered for .357 - you can do initial training with .38's, then carry it with .38 wadcutters, soft-alloy .38 hollowpoints (I like Winchester Silvertips or Federal Hydra-Shok,) or Glaser Safety Slugs. When she's comfortable, you can move up to .357 hollowpoints or wadcutters for "full power."
The .38/.357 is a nice compromise - if you get a .22, you're going to have to train on marksmanship under stress more intensively (if you carry a gun without the intent to fire it, you're going to get shot with it someday...) and the bullet is large enough to do some of your job for you. But, it's not such a heavy hitter that the average shooter can't handle it - unlike, say, the .44 Spl/Mag or .45 Colt/.454 Casull.