Get some 21g cartridges.
Put a good bit of time into getting her to stand and hold the gun correctly at home before you go. Make sure she can hold the gun without arching backwards to manage the weight. If she can't, then best get a lighter gun. First impressions count and you can put her off easily..
If it makes sense to her, explain that the correct stance is like a boxer about to throw a punch, NOT like a boxer receiving a punch..