Nicki has covered the single most important consideration........start with a 22, and practice, practice, practice.
A double-action revolver is the most practical handgun, in that its the easiest to use and simplest in operation.
A best quality one, S & W or Colt, is not cheap, but 'theres no free lunch'.
Have her try the feel of the various small frame versions, different barrel lengths, for 'balance', and look at the varieties of optional grips available......the most important thing in that context is to find the combination of handgun and grips which will 'feel right' to her.....it may feel 'all wrong' to you.
The double-action pull of many revolvers is 'too heavy' cos its not set up correctly. Having a revolver 'tuned' by a good specialist gunsmith will make the DA pull easy and smooth.....the techniques for doing this have been well refined, over the years and its relatively inexpensive to have done.
Nicki is technically correct in preferring a 45 as a self-defence handgun......but.....a 22, with maximum load hollow-point rounds, is a serious weapon, and will be a realistic choice for self-defence in many, if not all, potential scenarios.
A human assailant, struck in the neck or face, or even in a shoulder, by a 22HP, is exceedingly unlikely to continue an assault. A mountain lion, wild pig, or a vicious large dog of the shepherd/pit bull class, would be a very different story. A 357 or 45 is 'just barely adequate' against a dangerous animal.
The point, of course, is that if she is sufficiently interested to be willing to practice, she will develop the skill to be able to 'point shoot' quickly and accurately. Hitting with the 22 is far preferable to flinching and missing with a 9MM or 45.
This is where the humble 22, and a few thousand rounds of dedicated 'muscle memory' practice, will make 'all the difference', as compared to a heavy handgun which is too fatiguing, in terms of noise and recoil, to allow for comfortable practice.
So-called 'muscle memory' is the 'key' to effective use of a handgun. As an example of muscle memory, as any reasonably good guitar player to do a chord....and then a key of chords......and then the 'inversion' of those chords, up the neck......the guitar player doesn't have to think about where to place his/her fingers on which strings, at which frets.....he/she just thinks, for example 'Em7 chord at the 9th fret', and the 'muscle memory' response is 'automatic'. It took that player quite a few hours of practice to develop that 'automatic response', didn't it?