The guy should have taken an extra two seconds? Have you ever been physically attacked? You can get punched three or four times per second by someone who knows how. I don't even have kids, but if a child is struck by a strange adult, and the parent is not already aware of the person's mental disability, no one is going to wait around to see if the attacker is going to keep striking.

Yeah the Dad "could" have waited around to see if this strange man was going to continue battering his child, but that's not a response you can reasonably expect from any parent. Dad was completely justified.

As for how the handler should react, the answer is very carefully and peacefully. You would probably not legally be justified in striking the dad unless you had already made several peaceful attempts to explain, been ignored by the father, and the father is still attacking. Basically if the Dad was legally justified in defending his kid, and you attack him, then you go to jail for assault and battery. The mental incapacity of your friend only protects him, it does not protect you.

Ultimately I agree with you though. It was an overall shitty situation for everyone. Just because you are completely justified in punching a mentally disabled person in the face, doesn't mean you will feel very good about it afterwards.