I got to wondering because the wife turned almost turned the dog on a random stalker yesterday
The law considers the threat and disparity of force. Your wife is at a disadvantage to the stalker due to women are generally weaker than men and usually smaller than their attacker. The use of a dog for self defense would be considered reasonable. If a man that is 6'2", 245 lbs, used his dog against an unarmed person that is 5'7", 150 lbs, then it would not be reasonable because the 6'2" guy normally would not be in life threatening danger from the smaller person. Same is true for using a firearm, knife, baseball bat, etc.