Mossberg 500 youth model then. You might even be able to to get a combo model with both a smooth bore and cantilever rifled slug barrel. See if you can find a gun shop that carries the youth models in stock and let her shoulder one. I looked at the mossberg 500 and remington 870's. I liked the 500 better as far as fit goes. I looked at a browning pump that was pretty nice as well, but it was a bit too much for me and too pretty to hunt with. I think it also had a bottom eject which I don't care for much.

And like I said, the mossberg has a ported 12 gauge barrel. It helps a little with recoil, but worked great at reducing muzzle flip.

You can also get a .50 muzzle loader barrel for it.

By the way, I am 5'3" and weigh around 120-130 lbs. I had the youth model and bought an extended modified choke and a hastings turkey choke for it. I also put the True Glo clamp on fiber optic rifle sights on mine. It comes stock with twin beads though. I could go from bunny/pheasant to turkeys in a few minutes by swapping chokes and ammo. And they come with a IC, modified and full choke so you could shoot the brenneke's out of it as well.

http://www.walmart.com/catalog/product.do?product_id=3163656

http://www.mossberg.com/bantam_models.htm

Paul