BC, I assume we are talking about a smaller school district. Less than 100 kids to a class?
I would watch it play out as Blanch says . AND I would contact a school board member you or your husband have a relationship with and know to be a decent person that cares. Most school board members do, but not all.
Having been a school board member myself, I can tell you when I parent called I listened and got involved, or at least check on the "progress' of the situation. Schools are made up of people, teachers, Principals and other staff that want to take the path of least resistance. I can assure you when I walked into the classroom/office or wherever I wanted to go to "discuss" an incident. People knew the path of least resistance was closed. Also you should know sometimes the complaining parents kid was the problem, I felt it was my duty to mention that to the parent . However, that does not seem the case here.
Even if the parent just mentions it to a board member and wants the member to just monitor the incident and not get involved, the members will all know of the problem kid for next time and make sure it plays out fairly. There are certain advantages to living in small school districts. That is one. It has been 20 years since I was on the school board but I doubt things have changed much.
One cute side note to this. Our superintendent made a rule school board members could not enter a school with a "pass" from him. We changed said rule and he found a new job shortly afterwards. Never forget local politics when you need too use them.