Everyone who has recommended a .22 LR pistol is absolutely correct!
To back this up, the .22lr pistol is the most commonly used weapon for assassinations! Assassinations are usually an up close and personal situation, very much like the majority of home defense situations. If an assailant is far enough away where a .22 lr would be ineffective then he probably isn't even in your house and most certainly it's a better option for your family to flee instead of opting for an OK Coral style shoot-out.
In the late 1970's there was a string of mob assassinations, primarily of FBI informants and the victims were all killed with .22LR pistols. Mark Schorr wrote an article about these killing and the Central Intelligence Agency includes this as recommended reading for it's operatives, it is published on the CIA reading room website. Here is a link to an excerpt of the article on the CIA website:
https://www.cia.gov/readingroom/docs/CIA-RDP88-01315R000300510002-4.pdf
Here is a short quote from that article:
In addition to the above information, the .22 is substantially quieter than a 9mm and, if you have kids in the house, a .22 going off in a defensive situation can be substantially less traumatizing for the kids. Put a suppressor on it (not legal in CA) and it will be no louder than your average staple gun. A suppressed 9mm is still loud as balls.
One more point I'll mention is that all US special forces organizations make available .22 caliber pistols which are used extensively for neutralizing guard dogs and sentries.
Lastly, as to which specific firearm to buy for her, if she's anything like my wife, DO NOT CHOOSE FOR HER! ;-) . Take her to the range, take her to the store, let her feel each weapon in her hand and LET HER CHOOSE the one that feels the most comfortable. Keep in mind, the heavier the weapon the lighter the recoil, also, try and steer her towards the better, more reliable brands. Hope this helps.
PS. I personally believe the Walther P22 / 3.42" is an outstanding weapon at a bargain basement price. Let her try that one on for size. See:
https://www.sportsmans.com/shooting...tte-black-tenifer-pistol-101-rounds/p/1411363