Should be denounce one whom he saw eating by mistake in Ramadaan?
Whoever sees a Muslim drinking or eating during the day in Ramadaan, or doing anything else that breaks the fast, has to tell him off, because doing that openly during the day in Ramadaan is wrong. Even if a person has an excuse for doing that, (he should not do it), lest people dare to do openly things that Allaah has forbidden, i.e., openly doing things that break the fast during the day in Ramadaan, claiming that they have forgotten that they are fasting. If the one who does that openly is sincere in his claim to have forgotten, then he does not have to make up the fast, because the Prophet (peace and blessings of Allaah be upon him) said: “Whoever forgets that he is fasting and eats or drinks, let him complete his fast, for the One Who fed him and gave him to drink was Allaah.” (Saheeh, agreed upon). Similarly, the traveler should not eat or drink openly among residents who do not know that he is a traveler, rather he should do that in secret so that they will not accuse him of doing something that Allaah has forbidden and so that he will not encourage anyone else to dare to do that. And similarly, the kuffaar should be prevented from openly eating or drinking etc. among the Muslims, so as to prevent anyone taking this matter lightly and because they are not allowed to practise their false religion openly among the Muslims. And Allaah is the Source of strength.