Assault is the credible threat to commit harm
Exact definitions vary as laws vary state to state. A better definition of Assault would be intentionally (knowingly or purposely) causing a reasonable apprehension (determined by the persons present apparent ability to carry it out) of imminent harmful or offensive conduct.
I would by contrast define battery as intentional harmful or offensive contact.
As to the question of the OP:
Again exact laws vary but in general a person may use reasonable force to prevent a threatened battery on the part of another. This is typically true even if there is in fact no real threat; you are allowed to have a reasonable mistake. What is important is what you reasonably believed. Reasonableness is judged by an objective standard. The level of force used is another consideration. Generally speaking one may use force reasonably believed to be necessary to defend from the imminent battery. Conduct has to reasonable to stop what the threat is. Again reasonableness is measured from an objective standard.
If the person has genuinely disengaged, then self defense is no longer on the table. Actions taken in retribution are different from those taken in self defense. It is even possible that the initial aggressor who has clearly stopped being an aggressor could then have a right to self defense against a person who came after them.
Now we could think of hypotheticals and make arguments about when those elements have or haven't been satisfied all day, but those are the basic principles at work. There are of course issues of what the people investigating will believe happened and then issues of prosecutorial discretion (but remember civil liability and remember it can be easier to injure someone quite a bit more severally in a fight than was intended e.g. one punch causes them to fall and hit their head very hard on the cement causing permanent brain injury or even death). There are also issues of proof and what a jury will believe. Those may weigh in against what is viscerally satisfying to do or what seems morally justified. Legally speaking however, one would be better off to call the cops.
Since some have said they would try to beat him up and then turn to a gun if it escalated. I would remind them that if one is deemed the aggressor in a conflict (which if the initial aggressor had truly broken off that aggression and was no longer acting as an aggressor you arguably would be the aggressor if you proceeded to attack them) then that precludes an assertion of self defense. In sum if you go punch the guy and his buddy jumps in and you then pull a gun and shoot one or both of them and try to argue that it was self defense and the level of force was reasonable based on a disparity of force you are going to have some serious issues.
As a practical matter a physical confrontation may turn ugly and you may never have a chance to then get to that gun. I've seen enough one punch KOs (including sucker punches/shots with objects from intervening 3rd parties) or very quick beat downs to know this possibility is a reality.
I have over a decade experience kickboxing, wrestling and doing BJJ. I am not particularly afraid of getting in a fist fight with your average Joe. I will say from the following perspectives: legal (both criminal and civil), and a survival/strategies and tactics perspective, you are better off calling the cops. There may be fleeting satisfaction in teaching some punk a lesson but there is potentially a lot to be lost if one miscalculates or things spin out of one's control. Before I get in any fight I ask if it is worth dying over. I also ask if it is worth killing someone over. I ask if it is worth going to jail over or being sued over. It makes it a lot easier to walk away with that perspective.
As The Bard so wisely put it, "The better part of valor is discretion, in the which better part I have sav'd my life."
