Smoker in front of school. What to do?

While picking up my kids from primary school, I witness an argument between a lady (20+) and an uncle (70+). The uncle was smoking near a grey bin, outside of the school compound. The lady told the uncle not to smoke citing a sign on the school fence that smoking is not allowed within 5m of school compound. Uncle retorted aggressively that the lady should mind her own business, he is smoking outside the school and is under the not sheltered area. Lady insisted that it is within 5m of school compound. Uncle was agitated, aggressive and dared the lady to call the police. There were quite a few bystanders, mostly waiting to pick up their kids. Noone intervened, not sure if it escalated after I left. My kids asked me what was the commotion about which prompted me to think about my own inactions. I feel guilty for not stepping up for the lady. I didn't have a ruler and not too good at estimating either. Uncle could be beyond the 5m of school compound, which means smoking there is not wrong. Lady maybe had good intentions since many students will be coming out to the area. Asking for opinions on what will you all do in this situation, especially since uncle gave the entitled, offensive, wanna fight kinda vibes.