Slip-on butt pads work only if the resulting increased length-of-pull isn't outside of the ball park. You might consider getting your wife a shooting shirt/vest with shoulder padding. At one time, there was a product for lady shooters that was a hi-tech undergarment pad that was worn by attaching it to a bra strap.
One way to tame recoil is to use some of the extra low recoil loads now available. Another was is to have the gun as heavy as practical. An easy trick: remove the plug and fully load the mag. When she shoots, top off the mag after each shot or pair. Note: this may violate the safety requirements of your range. As a safer option, put the shells in forward of the of the mag spring. Shot filled hulls work even better than live rounds. The idea is to get the gun as heavy as is comfortable.
How to learn to shoot a shotgun? My mentor, an All American shooter, made the comment that he spent a lot of time observing others. Watching different shooters, he picked-up on what worked and what didn't. Learning by observing the mistakes of others is easier than your own trail and error method.
I was introducing my neighbor to shotgunning last spring and there were two young men at the next field. They were using a manual trap, one would run the machine while the other shot. Neither appeared to have much experience -- sadly, it was a case of the blind leading the blind. As one of the young men loaded the shared gun, I commented to my neighbor that he was going to miss. He called pull, shot and missed. My neighbor asked how I knew he was going to miss. I said that the gun mount had been so bad, there was no way that he could hit anything but sky. Stargazer before you have you wife do any shooting, spend a little time having her critically watch other shooters. When watching other shooters, most folks make the mistake of watching the target -- you won't learn anything that way -- watch the shooter. After his finishes his follow-thru, then look look to see if there are pieces or a whole target hitting the ground.
Typically the shooters who seem the most comfortable with their guns shoot the best. Not to over-beat an old drum; but, proper gun fit is as important to a beginner as it is to a champion. If your first shotgunning is with something that fits you poorly and gives you a beating, then there's good chance you'll not want to repeat the experience.