A .38 special should do the trick with hollow point loads but as was suggested a shotgun would be better for a novice inside a house and a .20 gauge would be plenty. One thing I would advise, have a light installed on the gun so the wife can see what she is shooting at or what not to shoot at and you do not have a tragedy with a family member walking around at night and getting shot. I keep a flashlight next to the handgun safe next to my bed, it is one that I can access my gun in the time it takes to put my hand on it. I have been trained to shoot with a flashlight in my hand but for a novice it is cumbersome and would be easier to have a gun mounted light. Above all, take her to the range until she is comfortable with the gun and can shoot it well and then continue going to the range. Practice in the dark as well. A gun in the hands of someone who is unsure of themselves is a danger to them. In addition. Talk to her about having to shoot a person in self defense, make sure she is sure she can do it. Not everyone can and it that is the case the bad guy is just liable to wind up with the gun.