There's a difference between finding a gun that fits properly and shooting a gun that fits properly. Many smaller women (and larger men) won't find a factory stock that fits them. That's why, when shopping for a gun, you have to keep in mind the possibility of needing to have the stock custom fit to the non-average sized shooter. Some stock materials and finishes are less amenable to custom fitting than others.
Touche!
You have many options to fix the LOP (length of pull).
Go with either a Remmington 870 or Mossberg 500 and there will be plenty of aftermarket stocks (in wood or plastic) with varying LOPs.
Some adventurous people even cut their own stock to adjust the length of pull. This works best on a wood stock; it can work on some plastic stocks, but some are hollow. Be sure to put masking tape over the area you wish to cut to avoid knicking and chipping. Also the shape of the recoil pad might need to be adjusted with a sander which can be difficult and dangerous. New holes for the pad screws might need to be drilled also. If done correctly, the stock won't need to be refinished. If you tried this method and failed the worst thing that would happen is you would need to buy a new stock, which would fix the problem anyways.
Probably the best solution is to buy a shotgun from a local gun shop (instead of a large sporting chain like Scheele's, Cabella's, Sportsman's, etc.) who has an in-house gunsmith. The gunshop
should give you a discount on gunsmithing work, like shortening the LOP, if you are buying a gun from them. Choose a shotgun (with a wood stock) that your wife likes best while ignoring LOP issues. If you can find a nice used Browning BPS, Remmington Wingmaster, or other higher quality shotgun in 20 ga then go for it! Otherwise buy new.
FYI - old (pre 21st century) Remmington 870 Wingmasters usually sell for pretty cheap (I've seen them for around $150-$200) while still being high quality; the only real issue with them is the blueing and wood finish are usually very worn, but if you are going to shorten the LOP by cutting the stock and eventually get your wife another shotgun later I don't see how it is a problem. Some people purposefully buy these old "beaters" to fix them up by refinishing or changing the stock and refinishing the gun. I've thought about doing that more than once.
If you are really desperate to save money then the NEF Pardner Single Shot (NOT PUMP) Shotguns are of good quality, made in the USA, and come in 10ga, 12ga, 20ga, 28ga, and 410. The only problem with them is that they don't come with a recoil pad and they kick harder than a pump due to them being lighter weight. Just cut to the proper LOP and add a recoil pad or have a smith do it. I had one in 12 ga and sold it years ago which I now regret. Its a very good shotgun for a beginner. Can be had for around $100 NIB. The best deal in American made shotguns. Easy to clean too! Stay away from shells with aluminum bases, these guns will jam on those sometimes.