Just read the bold print if you don't feel like reading everything.
Anything with a good recoil pad will be fine. But the most important thing will be fit.
If the above criteria are met, ask her if she's comfortable with a little extra weight. More weight will usually result in less perceived recoil (at least that's what I found to be true).
But do not go overboard, or you'll end up carrying her gun.
Comfort is rather subjective. I am comfortable with my 870 Wingmaster, even with 3" slugs and a piece of crap recoil pad. However, my friend is only comfortable with a "comfortech" recoil pad.
Shooters are usually uncomfortable with the setup, not the cartridge.
Anything with a plethora of customizable features (recoil pad, stock, sights, replacement barrels, etc.) will be fine. Because you can find a way to make the gun fit your wife without having to spend a fortune.
I know you asked for specific shotguns, so...here goes (oh, and it's not in order of best to least, or vice versa):
1.
Remington 870 Wingmaster. If you are not allowed to use a rifle where you live, you can consider a rifled barrel. Also, I do not recommend the express line because in that price range, the Mossberg 500 is just as good, if not better.
2.
Mossberg 500. I would recommend synthetic (since the wood version isn't too heavy either), and then replace it with the Knoxx CompStock or just a nice recoil pad. Again, you can consider a rifled barrel.
3.
Ithaca 37 in 20 gauge with a nice recoil pad. I can accurately shoot it with one hand
. I would not recommend 12 gauge because lightweight + powerful cartridge usually doesn't benefit the shooter.
4.
Semi autos such as Remington 1100/1187, Beretta 39X, and anything else that gives you the option of swapping out parts. Benellis are fine too, but I've been told that they recoil more. I don't feel it, but...that's something to think about.
I have not purchased an O/U
yet , so I'll let others chime in on that.
Hope that helps
.