My two sisters (22 and 37) went to the range with me a few months ago. I taught them to shoot (in the younger's case, a long-awaited refresher) with the MKII and the P22 5". Though the P22 is certainly not as accurate as a MKII/III or a Buckmark, it has several advantages: small and easily changeable grips for smaller ladies, very light weight (to reduce fatigue during long sessions) and, of all things, a laser. I was skeptical about how much a laser would help, but I really was able to see what my older sister was doing wrong at first because of the laser. Also, the P22 is more ambidextrous than the other designs mentioned here. Finally, a good mod is available to turn the P22 mags into 13 rounders, which makes it a more viable carry piece should the need arise.
That said, I will be purchasing a MKII before I buy a P22, though both will be mine. I, however, have a lot more guns to choose from (a .22 handgun is about the only genre short of NFA weapons that I don't yet have) and have a lot more experience shooting than (I gather) your sister does.