Lexi, It has been touched on already about the over-penetration of Slugs and 00-Buck, this is 100% correct. When I was in the Army and we trained w/shotguns at CQB one of the favorite loads for a 12ga was #4-Buck. The reason for this is lack of over-penetration, 27 pellets per 2-3/4" shell and you also have to stop and realize that each pellet is the size of a .22 bullet (diameter wise). In a hallway, stairway or your back against a wall and with a cylinder bore 12ga the assailant wont get very far. The major reasons a person dies due to having been shot or cut is the amount of blood loss/trauma/shock. 12ga #4 Buck is wicked in this department.
Now I'm not recommending anything here just giving an opinion, cause nothing and I mean nothing can ever replace a persons training, knowledge of the tool at hand, skill level with the tool and the amount of practice they undergo. You have heard it before and I will repeat it now, practice, practice, practice. If she has never received formal firearm training then this should be the first step PERIOD, get training. Then determine what she wants and would be comfortable with using, handgun, shotgun etc. I'm sure the guys at Busse have enough guns laying around and they would be glad to let her shoot several different firearms. One of the biggest challenges people face w/guns is the fear of recoil. She needs to understand this, work with it and lean not to fear it. If she is uncomfortable with a specific firearm then she shouldn't use that one and move on to what works for her. But like I said TRAINING FIRST!!! You and maybe a couple others in the family could pay for a training coarse for Christmas, just and idea sweety.