I would recommend an auto-loading shotgun with a 26" barrel or perhaps a lighter O/U or SxS and use low-recoil shells. The O/U or SxS would probably get 28" barrels. These would be a good starting point, in my opinion, and then you could fine tune the fit from there.
The main issues my fiancee has had with my various guns has been the weight, the LoP, and the recoil. We initially purchased a 20 ga for her and I added a stock labeled for youth and women (as opposed to just labeled for youth), and then we added a recoil reducing pad to the stock. This seems to fit her fine, but the weight and recoil with standard shells are still an issue.
Her next shotgun will likely be one of the above-mentioned, but perhaps one designed to be light weight, ala the Citori Feather, one of the Benelli variations, etc. I understand that many prefer the feel of a heavier gun for recoil reduction, faster follow-up shots, and what have you, but, at least for my better half, if she can't hold it up after a few stands of clays, then she's either not going to be shooting any more, or she's just not going to be hitting much, so those things no longer matter, and her fun factor has significantly diminished.
The low recoil ammunition I've tried has basically no kick - it's actually quite surprising if you haven't tried it already. Additionally, the recoil is felt a lot less, in my experience, if the gun fits the shooter properly. Along those lines, if you're going for the 20 ga simply because of recoil, I'd say go out and shoot a box of Winchester or some other low-recoil 12 ga shells and see what you think before you make the commitment. In all honesty, I believe I'll be getting the lil' lady a 12 next so she can have the benefit of extra pellets and the 12 ga low-recoil rounds are much easier found than the 20s in my neck of the woods. If, on the other hand, the lady simply prefers the slimmer and quicker feel of the typically 20 ga, and the balance it offers as between the perennial 12 and the rarer sub-gauges (like myself), then I suppose this point is moot.
Best of luck.
Gunr