If she is comfortable with it and shoots it well, anything from about .38 special up would be fine.

If she is a country girl with a lot of shooting time and good safety skills, the training that must be ongoing will be easier and less tedious. If you have to start from scratch - use it as a set of test driving sessions. Rent a lot of different pistols at various ranges and let her shoot them and decide which type and later which model she likes.

Very important - start with a rimfire pistol and move up slowly from one caliber to the next. My wife started with a .22 revolver, moved to a .38 revolver and settled on a 9mm auto with specific mods she wanted after shooting everything I could lay my hands on. I have never mentioned or discussed recoil or muzzle blast. I have had extensive discussions with her about velocity and energy. She routinely puts ten rounds from my .357 sig into 1" at 75 feet without realizing that most people won't even fire that pistol due to perceived recoil/blast.

My wife has a slightly modded glock 26. Not so awful trigger (glock competition trigger package + 25 cent trigger job) and a +2 mag extension.