I'd take her along and let her pick it out. That will make it hers, and she'll like it better.
I think 21" will work fine for her. The short barrel is too fast for a lot of men (except in the brush), but that's what makes it great for her.
The Remington 870 or 1100 Upland Special would work fine, and so would a Browning BPS or Gold. I believe youth models are made for the Remingtons, and I'm pretty sure the Brownings come in "micro" configuration that is short and light.
Remember that AAs come in various charge weights. You can shoot skeet and 16 yard trap just fine with the 7/8 oz. "Low noise, low recoil" 12 ga. loads and recoil is minimal in a heavy gun. The Browning BPS 12 ga. Upland Sp. weighs about 7#, but it's short enough she might like it. The 20 ga. BPS US weighs about 6.5#.
A Beretta 391 20 ga. is an awful nice gun too, for a little more money. If she likes it and you can raise the money, the chance of having a live-in hunting buddy would make it a bargain.
Whatever you do, be sure she likes the gun before you buy it. Women often like pretty guns. I helped a friend select an SKB auto a few years ago. It is a 12 ga. slug model, with rifle sights, short stock, cylinder bore, and "engraving" of birds on the sides of the receiver. She loves that gun.
On the other hand, my father in law went to town to buy his wife a .243. He found a good deal on a .308 BAR and brought it home instead. She hated that rifle and never, to my knowledge, ever fired it. I hunted with it some, and it was a hell of a rifle, but not what she sent for.
I'm not an expert on guns or on women. This is just what I'd do in your shoes.
