Oh, I was going with the presumption that the husband is unarmed at the time of the incident. If I'm armed it's over. I would take immediate control of the situation without a punch being thrown (*probably*, depends on the situation)
If I was unarmed and he was bigger than me, it doesn't matter, enter the fray with all manner of enthusiasm. I may get the worst of it overall, but he'll know that he was in a fight afterwards. The goal is to protect the lady and win/lose.draw it can be accomplished with fisticuffs. The attackers focus would go off of my lady and violence towards her would cease. She could retreat somewhat for safety and call in the boys in blue while I'm dealing with the attacker.
I actually had a similar incident once. I was walking in the neighborhood and heard yelling. A man and woman came into view from around the corner and the man was dragging her by her hair and slapping her around while they walked. She was yelling for help. I of course went to them to see if I could help (I was armed with a pistol) and I approached and said do you need help to the woman. The man began yelling at me that this was his wife and to mind my own beeswax. (Strong odor of alcohol, they were both drunk). I do not like to get in the middle of a man and his wife, who knows what the real situation is, too many variable) Nevertheless, violence is not to be tolerated and she was looking pretty beat up with marks on her face and such. So I looked at the woman and said, do you want help? She said yes please. He was about my size and drunk, I wasn't drunk so didn't feel the need to draw a weapon. So I punched him in the face a couple times and grabbed him by his long hair and took him to the ground. He wouldn't let go of her hair so I punched him a couple more times in the face and told him to let go of her. He wouldn't so I repeated the procedure of punch him a couple times and order him to let her go. I probably had to punch him 10-12 times before he got it through his drunk head that he better let her go. Eventually he released his hold on her hair and she began walking away without another word. (I think she did say thank you). I held him down for about a minute more to give her time to put some distance between them, then released him. He was bleeding a lot and still shouting obscenities at me but turned and began walking away. She had made a corner by then so I felt it was over and went on my way. I didn't call the police. Even though I was armed, the thought of drawing never entered my mind. It wasn't necessary. I am very familiar with weaponry and am very practiced up on draws and know the local laws very well. Drawing a weapon for me is...automatic at a certain point of threatening situation. I don't need to think about it. I didn't need it in this situation, so it never left the holster. I didn't even think about the weapon until later I realized that my instincts and prior training (practice) (or the Spirit) led me and took over. (another time a dog attacked me and my friend and I had to fire a warning shot into the grass to turn the dog off of his charge, which it did, and later, my friend told me that he never seen anyone draw so fast. I didn't think about that either, I just did it. So it is true that in a stressful situation that one automatically reverts to the level of their training.)