I'd think the new Glock 44 in 22 LR would be a good choice. It's dimensionally identical to a Glock 19 9mm. She could practice with the 44, then keep a 19 around if she actually needed to shoot somebody. A revolver and a Glock are identically simple to fire. Keep your finger off of the trigger until you're ready for it to go bang.
I was at a gun store today (without the wife) and handled the Glock G44 in .22 caliber. At $400 it had my butt cheeks tightened at the thought of spending my toy money for it.
I asked about a hammerless revolver - several here have mentioned it, and like UPhiker just stated, the salesman was against the revolver for a woman for exactly what UP said. He said the heavy pull would cause her to jerk it to the side and miss the target.
The Glock G44 .22 caliber is the leader in the clubhouse at the moment - I’ll have to drag her over to handle it.
