20's are good for anyone for the lowered recoil. Leroy Thompson wrote a great article on the fighting 20 gauge past and present a few years ago. Here are some classic defensive 20 gauges:
Ithaca Auto & Burglar
I believe a Remington M 17
Remmy Model 11's, the Winny is in 10 guage
For pumps today you have Rem., Moss., Win., and Ithaca. An 11-87 would be a great choice as auto further dampens recoil, and on that note, have to bring up the Saiga 20. 8 round mags. Even with the ugly fixed stock this is a devastating defensive weapon. Then there are the SxS coachguns, hammered and non-, cool guns but far too low capacity and slow to reload. I'd say a Saiga, but an 11-87 would offer a little more class and smoothness. Bonnie Parker loved her Remington 20 ga. auto, there's one female endorsement.
Build it up with 7 rd. Extension like BoltWorks did, Ghost rings, synthetic stock, etc. similar to the Wilson/SGT K-9. But it's pretty hard to argue against a 8 round shotgun magazine (which go for over a hundered, the only downside). If a pump is called for, an 870 or Persuader/Cruiser (whichever is the stocked version). I don't know much about autos other than 12's, I know you need a higher power load to cycle reliably, but the system soaks up recoil better than anything. Of coarse, a auto will never be as reliable as a pump.
Also she could look into Vang Comp barrels, maybe that would let her handle a 12 even, but would make a small 20 ga. suitable for a small child, bring it down to .410 comfort.
I hope she enjoys it, I love the 20 gauge for defense. #3 buck is great stuff.
one more pic, a fixed stock/with PG Saiga. I like the original folding stock, but like I said, even the version with the one peice buttstock is awesome for horrific capabilities.