I bet the guy gets arrested.
More likely he'll be sued for punching the thief more than once. And depending on the jury, he very well might lose and have to pay big bucks to the thief.
A good (that is, unethical) liability attorney would just have to get the thief to say that her jaw now hurts all the time.
And then show the jury the video, emphasizing that the second citizen told the first one to stop punching.
Yep, with the wrong jury that man could be in trouble. No good deed goes unpunished.