Whoops. Guess who is now on trial for manslaughter?
Well it all does depend. I recall in California law even someone that is a mutual combatant can regain their right to self-defense technically if they try to stop fighting and clearly communicate that.
In reality it probably won't work for them most of the time, but for a third party defending them it could depending on the situation.
If they try to run away, stop fighting, are pursued, and the other person is intent on doing them damage, they can regain their self defense rights.
Also if you witness a fight where one guy is knocked out and goes down, and the other guy begins to stomp on their head, there is a real risk of serious/great bodily injury or death.
Police or authorities acting in a lawful manner also don't generally stomp or beat on people unconscious.
The fight just went from a fight to something much more serious legally.
Boot to the head is often even considered use of a lethal weapon. Injury likely to cause or that is causing damaged teeth, eyes, skull etc can qualify as serious bodily injury and can quickly go past the gray area and often allows for use of lethal force, including by a third person.
Now you may want to wait until after they stomp on their head at least once if it is not someone you know because the injuries help establish evidence of the seriousness of the situation, but once the stomping starts you can use whatever force is necessary to stop it.
You may want to warn them, try to stop them verbally, but you may also need to shoot or hurt them rather quickly to keep them from killing or permanently disabling that person. It doesn't take long to permanently disable, cause brain damage, or kill someone down on the concrete, a few seconds can be the difference between needing some stitches and being a brain damaged vegetable or severely disabled for life.
Additionally if one person goes uncouncious it becomes quite easy to establish that the other person is acting in an offensive manner if they do not cease further attack, and continuing to pose a serious threat to someone unable to defend themselves.
It is a gray area depending on the severity what is and is not 'serious bodily injury'/ 'great bodily injury' but if it is causing some serious injuries or may kill the person it can qualify. Trying to inflict injury to anyone unconcious pretty clearly qualifies in most cases.
So I would normally not interfere in what seems like a mutual fight. Or even a one sided attack where the victim could easily get away yet obviously is choosing to remain and just contributing the to situation (like in many domestic situations where someone is slapped or punched etc and continue to argue)
However if I see someone being chased and subsequently beaten down by multiple attackers, or I see someone go down in a defensive ball or unconcious and continue to be attacked by one or more attackers I may choose to intervene and deploy the force necessary.
While warning shots are normally best advised against, it is also one of those situations where they can be considered (of course you are still responsible for where the bullet ends up) before shooting someone who is a clear aggressor in an altercation you are unsure of. It may stop someone who is using force likely to create or creating serious bodily injury, without requiring you shoot them, or it might not and require your next shot to drop them.
But it has more validity in defense of a third person than in self-defense, because it can actually be done at a point where if you were the victim you wouldn't even be able to deploy a gun anymore and the severity is more established.