I'm not sure if this is a genuine topic question or a fantasy, but I agree with many here, and if it's for real, this is what I think - You may enjoy the fantasy and your wife may want to go along with that and even find it exciting too, but if at all then it should only happen within that context! An arranged wrestling match for example should be ok for everybody, and then it's not going to be a disaster if your wife doesn't win.
But to encourage her to provoke some slut who doesn't know what's really going on and would just enjoy giving a black eye to another woman would be wrong. If I was in that situation, a woman hitting on me in front of my wife/girlfriend, I would shrug my shoulders, take my wife in my arm and kiss her. That should be 'victory' enough for her - she wouldn't have to stand up for herself at all, because if we love each other we would do that together.
The fantasy of her fighting would be completely different thing - that could be arranged if both want to do that, but not in a random way where in an uncontrolled brawl she could just be injured and emotionally really hurt.