She has no experience with firearms but I am leaning her towards a 20 gauge pump not worried so much about stock length because on a cheap gun a hacksaw can do wonders I am trying to talk her into going with me to a gumshow next weekend so she can get a feel of a few guns.
Perhaps not as easy as you think. Cutting a synthetic stock with a hacksaw can be a problem. Perhaps easier with a Mossberg than a Remington 870. A wood stock is easier to shorten. You may then have the cost of a new recoil pad.
I don't know if it is such a good idea to get her a pump of any kind. Modern auto loaders are pretty reliable and you always have to worry about short shucking a pump under pressure.
If recoil is going to be a problem, she is probably better off with a gas operated semi. Some 20 gauge pumps are built on a lighter frame than the 12 gauge and the reduced weight results in less reduction in felt recoil than you might expect. Of course it may easier to find a pump shotgun with a HD barrel (short like 18 to 20 inches).
While the pump tends to be less expensive, it's a waste of money if your sis isn't going to practice with it and feel comfortagle using it for defense.
How tall is she?