The punch might have been a bit justified, because the guy picking the fight was being a bully. However, a different tact might have been more prudent. You can't just sucker punch someone because you don't like the way they are acting.
It all depends on what you mean by "how they are acting."
I agree you can't sucker puch someone for wearing a silly hat or telling jokes you have heard before.
However, if you use reasonable force in the protection of a third party it is justified. It was obvious someone was about to get hurt. Why should that someone not be the guy who simply will not take, "I don't want to fight" for an answer?
The first kick while the bully was on the ground was borderline, but I would give the kicker the benefit of the doubt on that since he may not have known instantly that the guy was no longer a threat.