I suspect it is a combination of two things. An ill fitting shotgun and the fact that rifled slugs produce a different kind of recoil than rifles, it is more of a whack than a push. I would not shoot fine shot through a rifled barreled shotgun. Perhaps getting the gun fit to her, or a good recoil pad would help, having the gun fit would cost more but it would be the best and permanent. Or, perhaps you might want to think about low recoil rifled slugs, they do reduce recoil substantially while having more than enough speed and weight to cold cock a deer.