I would echo what other people said about more common ammo. Once you drop below 5.56 you are looking at short range carbines. Maybe something in a pistol caliber would be better. A pistol caliber that matches other guns you already own.
Another option might be stripping down a short AR to make it lighter and training her to get used to the recoil. You can't get much lighter recoil than an AR, but a pimped out AR with a metal forearm can get pretty heavy to lug around. If she was shooting an AR it might not really be the recoil, but the racket from the recoil spring right next to her head that she doesn't like.
My wife has trouble holding an AR up, she also has trouble with the slide of an automatic, and the hammer of my ruger .357 revolver.
I’m thinking of a 20 ga double barrel for her.
Good point about the recoil noise. Get a “twang buster” or look at advabced alternatuve stocks.
>>If she was shooting an AR it might not really be the recoil, but the racket from the recoil spring right next to her head that she doesn’t like.
That’s a really excellent point that driftdiver ought to have the friend carefully consider and explore. Should be easy to overcome, if that is really the root of the problem.