Nothing wrong with an 870 at all. Shorten the stock to fit her and get one with a shorter barrel or shorten the one you get. I suggest a better recoil pad as well.
My wife had fun with my 870 with pistol grip stock and a 20" barrel shooting some Aquila mini shells. I did not modify the gun to run those shells and that was a bit of a pain but she had no issue with tamer full sized shells just had more fun with the mini shells.