Her hands are too small to confidently grip full size pistols or work the slides.
The issue you are running into in terms of size is the grip circumference, not grip length. Don't confuse the two. The problem is really double stack versus single stack, not long versus short grip. She will have more problems with a gun with a short grip (PM9) than a gun with a longer grip (P9 or K9).
The short grip guns like a PM9 and MK9 only allow you to get two fingers onto the grip -- your pinky dangles. This makes the gun more difficult to control, particularly for people with weaker hands. She will likely have a much easier time with a gun that she can fit all of her fingers onto the grip.
Furthermore, the recoil spring on the PM9 and MK9 is significantly stiffer than the recoil spring on larger guns like a P9 and K9 -- since the slide on the shorter gun is lighter, in order to control the slide speed they must have a stiffer recoil spring.
Her problem lays with physically countering the recoil spring to manipulate the slide, as to chamber a round or even more so to lock the breech open. I tell her it is just a matter of practiced technique and some strength training but she is frustrated with the inability -- which is understandable as we all want to do well at a thing we attempt, particularly if it is something new and a little intimidating to begin with.
She simply won't be able to lock open a Kahr PM9. She won't have the strength. A P9 or K9 will be a little easier, but still hard for her.
Also the MA version of the PM9 would give her the addition of a manual safety if she wanted to use it (and train with it).
You really don't want this. Seriously. The manual safety on the PM9 is small and hard to operate under stress. It is only there for compliance with the AG's regulations.
I own four Kahrs, including the PM9, MK9, K9, and K40. If you and your wife would like to try them, you could meet me at Harvard Sportsmen's. I will tell you up front that, in my opinion, the PM9 and MK9 probably won't work for her. The point of the PM9 and MK9 are to make the gun as small as possible so it will fit in a pocket. As a result of that smaller size, you have a gun with a stiffer recoil spring, and a shorter grip that is harder to hold. Unless you are going to carry it in a pocket, then it really doesn't make sense to make those compromises.
The P9 or K9 would be a better choice, but she may still have trouble dealing with the slide.