Do NOT make her use your shotgun! If the shotgun stock is too long, it beats up a woman even worse than a man; you may have noticed they are physically somewhat different in the upper body area. It is
critical that the stock fit her. If ya need to buy another buttstock to cut short for her, do it.
When she shoots, encourage her to lean forward into the gun. If she leans back (which for some reason new shooters tend to do), her shoulder and back are gonna take the recoil, rather than her entire body.
Use light loads. Winchester AA12FL8, "Low Noise, Low Recoil" is the way to go. They are called Featherlites, but the only indication of that is a feather on the box. Winchester also makes a non-reloadable WinLite, but I've never seen them on the shelf. The Featherlite is .92 oz. of shot moving at 980 fps, which is plenty for SASS. I reload using the Winchester Featherlite recipe: 12 grains Winchester WAALite powder, 26 grams of shot (.92 oz), Win AA12SL pink wad in a Winchester AA hull. Works great, and has no problems taking down knockdowns. The Winchester ExtraLite (AAL128) is a tad hotter load - 1 oz of shot at 1180 fps. My wife uses those for skeet.
Another + for the 12. Unless you load your own, 20 gauge light loads are next to impossible to find (although several manufacturers catalog them). Also, the 20 is usually built on a slightly lighter frame, which accentuates the recoil. Most folks I know that started with 20s to try to get light recoil have gone to 12s. This includes men, women, and even some kids.
A friend of mine with a pacemaker tried a recoil reducer in the stock. He found that with the light loads we shoot, there was no perceived improvement in felt recoil. He would have done just as well to put a dead weight in the stock.
If your wife wants to talk with another lady shooter, message me, and I'll have my wife get in touch. If she'd like to see some websites about lady shooters in SASS, my wife's site is
http://www.NovemberTriggerSisters.com , or another popular ladies' site is
http://www.carolinabelles.net/