You observe and make a judgment.
And heaven help you if you miss the perp and hit a bystander.
Just because you are carrying doesn’t mean you are bound to respond.
It could get you killed.
The scenario you have described, defense of another in a public place is the trickiest situation there is.
I don’t know why we are even discussing this. Just post a “no guns allowed” sign and this would not happen. Just ask a Liberal. All we need is more laws. (s)