I agree with Mountaineer.

I would've picked an 870 Youth (20 ga) for my wife, but fortunately had her along at one point and she "tried on" the 870, the Nova, the Bantam, and the BPS. Watching her, it was clear that the longer forearm of the Nova was significantly more comfortable for her to reach and manipulate comfortably. Even a 20 can get a smaller-statured shooter's attention, and slugs are just going to amplify that effect. I'd be very leery of using buckshot if you're serious about her shooting out to 100 yards, though.

I'm assuming you're in a "shotgun only" area, otherwise I'd suggest you also look at a few rifles, maybe something in the .243- to 7mm-08 range. My wife actually finds the .243 louder, but worries less about it breaking her shoulder than she does the 20 ga.

Her biggest complaint is probably that neither of the twocome in pink!