I think I see your problem Brand New. You have some great advise from experienced and knowledgeable shooters, but your Wife must feel comfortable when she goes to try a gun in the shop.
The problem may be that when most new shooters pick up a gun for the first time, they tend to very much favour lighter guns. This is true of male and female shooters. Therefore, particularly with your Wife's previous injury (which is bound to cause some apprehension) she's almost guaranteed to find the 20b most attractive whilst in the shop.
Of course, as you will know the lighter the gun the more recoil will be felt (ignoring semi Auto's of course) So for example a standard speed 28gram cart through a 6.75lb gun will kick somewhat harder than the same 28gram load through an 8lb Sporter. This is true pretty much regardless of the size of the bore.
So the dilemma is presumably - which is the harder thing for her to become used too, a slightly heavier gun or sharper recoil?
To some extent only she will know that, but please do bear in mind the only way she's really going to know is by using gun weight and cartridge load/speed combination that she's comfortable with. Is there anyone who would perhaps lend you a light 12g or a standard 20b to try? If not my advise would be to contact your local club and book some time with the instructor. that way she can try the options for herself.
Having said all of that, my vote is also for a decent quality slightly lighter 12g gun and Hull Comp X 21gram carts to start off with. Trust me, the Comp X are VERY low recoil.
Finally, someone mentioned the Caesar Guerini "built specifically for Ladies" These are called Syren and are indeed super bits of kit. However they cost about £2400 new and are not exactly lightweight, so perhaps not ideal?
Really hope you get things sorted, and good luck!!