A shotgun shopping trip, with her making the decisions, asking the questions, and you writing the check, is in your immediate future. She should have the shotgun that fits her, that she can handle the recoil for, that matches what you want to do.
You can't buy one for her. She has to do this for herself.
I think, in some ways, this is even more true for shotties than for pistols (and anybody who's been around here awhile has run across me opining on Let Her Pick It Out Herself on pistols and revolvers.) Because if handgun recoil is a mouse, some shotguns' recoil is an elephant. And it hits against areas that are, um, tender and do not take well to bruising (voice of experience here.) VERY tender and VERY much don't like bruising....
That said, I will tell you where I started in my search for a good shottie, and what I ended up with. I originally thought a 28 gauge would be about all I could manage. I have very thin shoulders, they have rheumatoid arthritis affecting them, and then there's that whole Very Tender Bruising thing. Anyway, I looked and looked and eventually picked a 20 gauge semiauto...a Beretta Urika 291a. Even with a semiauto, though, I found that I had to have the stock cut down to accomodate a very good recoil pad installation. With that recoil pad, I can shoot that Beretta all day long if I could afford to
But that's me. She needs to pick her own. Some women are small enough that a youth model works for them. Others complain that a 20 gauge youth kicks like a mule and they hate it. For me, at 5' 7", a youth model felt like a toy, so I got a regular sized Beretta.
Take her shopping. Have fun.
And tell her "congratulations" on getting into this. They're a hoot.
Springmom