This is your mom we're talking about right? You're getting advice from males. I'm a male too, but let's think from a woman's perspective.
Men are concern about gun size based on a man's pants pocket. Men's pants pockets are generally large, while woman's pants pockets are smaller.
WOMAN DON'T NEED BIG POCKETS BECAUSE THEY HAVE PURSES!!!
That said your mom still needs a small .22lr pistol. She can carry something bigger than what fits in your palm because she has more room in her purse. Obviously a Ruger MkIII is way to big. Many people were putting down the Walther P22 based on it's size. It is not a good pocket pistol, but it is a excelent purse pistol! Not saying you have to get a Walther P22, but you can go larger than the typiclal guy's pocket pistol.