Nothing beats a shotgun for entertaining uninvited guests in the home. Get the shortest barrel and most open choke you can. Don't load it with buckshot - use bird shot. She'll be able to clear the bedroom with her eyes closed... And not have bullets travelling through multiple walls and hurting kids in their beds. And as far as getting bad guys to pay attention and show reverent respect, I don't think anything can come close to the spectre of a freaked out woman pointing a shotgun their direction. Doesn't even need to be pointed "at" them, just anywhere within about 120 degrees of their position - they'll ask permission before shatting themselves...
- DAA
Agreed, if home defense is your goal, the easiest and safest by far is a shotgun with bird shot. Although, for a woman, that may not be the funnest gun to go shoot just for fun. I think about my wife and she'll go shoot my p22 most of the day, but even if I could get her to go shoot a shotgun (of any gauge) I'd get one shot out of her and then the stink eye the rest of the day.
IMO, it's always important to be comfortable with your carry/home defense guns, but it's more important to just be comfortable with guns in general first. Get her shooting something smaller first, then when she really does enjoy it, get something that's better for the home defense (like a 20 gauge).