They could have "gone after him" long enough to get a cell-phone photograph. Post it anonymously, notify the police, and get the victim to identify it. It's not that hard.
Go ahead, show us how it is done!
You get in your car and chase down someone who has a minute or two lead at the start, fire up your cell phone, and take a recognizable picture of the driver of another vehicle, while he is still moving. If you don't get pulled over for using the cell phone while driving, you will find that a picture clear enough to identify someone else in a different car is not as easy as you think.
And, the challenge I lay down is far easier than the challenge a good samaritan at the scene would face. There the perp would be actively trying to evade his pursuer, spoil any potential picture, and probably trying to run the following vehicle off the road.
Lay off the hero complex and live to fight another day.
I don't like the knockout game any more than you. But, I am not going to do anything stupid to catch one guy. And, I am not going to try to convince anyone else to take legally dubious action. Be aware of your surroundings, be prepared to win an altercation, but avoid confrontation if at all possible. OTOH, put pressure on law enforcement and news media to publicize this problem.