Thug, Wife, Unarmed
Adept,
Tellner was correct with regard to making an evaluation of risks in your neighborhood. Any scenario I can think of will require closing the distance to very close range.
Suggestions: A strong pepper spray. It will probably nail both the BG and your wife, and may affect you indoors. Close and disarm the BG. Of course, you would need to train for this.
Close quarters and indoors a Little League baseball bat can be a powerful weapon. Spray with pepper and clobber him! (Hiking sticks also fit in this category, may be easier to handle, but would not have the same force of impact.)
Some Taser imitators are on the market. I do not know how effective they are. You might be able to obtain the real thing which allows you to shoot the barbs from a distance. Again, the wife might be hit, but it is hard to hang onto someone who is writhing in pain!
A cell phone (or walking phone) might be very effective. Dial 911, keep them on the line, describe the situation and the perp. From reading some other threads, it seems some BG's are more afraid of a cell phone and 911 than the gun! At that point, reason might prevail. "The cops are on the way. Let her go and you can disappear before they arrive and you will probably never be caught. Take her, it slows you down, and they will take the city apart looking for you and her!"