If you are minding your own business and someone reaches from behind you and attempts to grope you, you have all the right in the world to fight back.
And it might be nice to say, “Hey, why didn’t you just ask him?” But if the other peson is intent on doing you harm, taking that much time may be all he needs to get a grip on you such that you are now helpless.
As for whether the guy’s head needed to be smashed over and over again, well, that’s a judgement call, and that’s what cops and courts are for. The law states that you have a right to defend yourself with commensurate force. What constitutes commensurate force depends on a whole lot of things. For instance, the law can and has upheld the right of a woman to shoot an unarmed assailant dead if it was clear that the assailant was attemping grievous harm. On the other hand, people have been convicted of manslaughter for killing other people with their bare hands. It’s all about who was bigger, whether it was clear that lives were in danger, etc.
But I’ll tell you - the people who fight ‘fair’ and give their assailants chances and all that honorable stuff are the ones who generally wind up being hurt badly. I watched a ‘fair fight’ one time in which a guy was attacked, and he put his attacker on the ground with a couple of solid punches. Then, he stood over him and said, “Had enough? I don’t want to hurt you.”. The bad guy nodded that he’d had enough, and the hero turned around and started walking away. The bad guy got up, took a run at him, and put both boots into the small of the good guy’s back - breaking his spine. Put him in a wheelchair.
Do everything you can to avoid confrontation. Never fight someone unnecessarily. But if you find yourself in a fight, assume that you are in serious danger and act accordingly. That means either running like hell if you can, or subduing your opponent so he can no longer fight you. This nonsense about hitting someone once and then stepping back to be ‘fair’ is idiocy. All you’ve done is re-open an engagement with someone who is now REALLY mad.
Dad would have looked pretty silly had he grabbed the guy, hit him once, and then let him go - only to find himself staring at a knife. In this case, it may have been a case of mistaken intentions, but sorry - Dad had no way to know, and anyone who tries to grope a stranger is the one putting himself in danger.
The people here who are shocked at Dad’s behaviour (even more the explanatory paragraph) simply don’t understand self defense and what it entails.