I think there is a lot of confusion about home defense. My first choice would be a secure room with a cell phone, and then access to a weapon. Having a gun doesn't solve the home defense problem, since most guns are in a gun safe and not easy to get at quickly, if you store it legally. It would depend on how quickly she can get to the gun. It really doesn't matter what it is. I have a 15-22, and if I put 2-3 shots in the head or the body, good chance the person will be down, whether a 22 or a 45 acp. The average person looking at the MP 15-22 doesn't know it is not a .223.
A pistol would might be a good/better choice. I keep a revolver in a bedside lockbox with fingerprint lock, so it can open quickly. THis is hard to do with a rifle or a shotgun without a bigger investment. A tactical shotgun it probably the best if you can get to it easily, but it not good for anything else, so if you are looking for something fun and functional I would get a 15-22 or a pistol/revolver.
I would suggest you find a personal defense course for your girl friend, and get her a dog. This is one of the best home defense things you can do. A lot of times you do not have a chance to get to your weapon and a dog will general scare them off before they even enter the house.
THe 15-22 does make for a great first rifle, and it is really fun and cheap to shot. I have one, and put 2-300 rounds thru everytime I take it to the range. I also have a shotgun I use for sporting clays, and I would consider it my backup weapon if I needed one and had the time to get it out of the safe.