What is the ruling on openly breaking the fast in public during the daytime in Ramadan for someone who has an excuse, and what is advised for someone who does that?

It is not part of the etiquette of fasting to openly break one's fast publicly, even for one who has an excuse, because their action might tempt someone who is unaware of their excuse to break their fast. A Muslim is a caller to Allah through their actions and words. It is obligatory upon anyone who sees someone openly breaking their fast to advise them gently and to explain to them, with kindness and wisdom, the effect of their action on people.