Dad overplayed his hand. An adult slapping someone else’s 12-year-old child is grounds for a beatdown.
You tell me and I’ll decide how to punish my child.
Most parents of bullies I’ve dealt with are also bullies. The kids learn it from somewhere. I have sorted it with the parents though and not the kids. In one case, I told one father that if his son hits mine again for no reason, I’d be coming after him hard. It stopped. I also tell my kids that they are allowed to defend themselves and others being bullied, but if I get wind they instigated there would be huge issues for them.