Here's what I'd probably do if I were in your shoes. I'd probably get the Rossi combo. I'd opt for the 12 ga (not the 20 ga) and .22LR combo.
I would get a set of chamber gauge reducers. You can get them here (
http://www.cabelas.com/cabelas/en/t.../search-box.jsp.form23&_dyncharset=ISO-8859-1 ).
If you are dead set on the 20 ga (less versatile), you can merely get the gauge reducers for that shotgun too.
Then I'd start her out shooting .410 - you'd be amazed at how nice it shoots out of a bigger gun with these adapters at least according to the panel of experts (also known as my sons).
Since she has a single shot and only needs ONE gauge reducer and they come in sets of TWO, I'd keep a set for yourself (for amusement, or whatever) as she only needs one gauge reducer for each size.
After she's thrilled (and not beat up from the recoil) with the .410, move up to .28 gauge, etc. She will tell you when she doesn't want to go with any more payload (and recoil).
It will be a nice way to get her acclimatized to a shotgun.
Also since she presumably has the .22LR barrel (the Rossi combo), she can have fun shooting that as well.
She will also have some other inexpensive options to try out with her 12 ga here (
http://store.dinaarms.com/ ) or here (
http://www.mcace.com/ ).
Just my 2 cents worth.
I hope you and her enjoy her new shotgun whatever you decide on.