The opinion that one should not remind the one who forgetfully eats or drinks while fasting in Ramadan is invalid. Rather, it is obligatory to remind such a person. This is because eating or drinking during the daytime of any day in Ramadan, without a legal excuse, is regarded as a sin. Therefore, one should remind whoever does so unintentionally based on the principle of enjoining what is good and forbidding what is evil. On the other hand, maintaining that reminding such a person is not obligatory will make those who do not fast dare to eat and drink in public during the daytime in the days of Ramadan.
Dr. Husam al-Din ibn Musa `Afana, professor of Principles of Islamic Jurisprudence at Jerusalem University, Palestine, states: This invalid opinion is the result of misunderstanding the hadith narrated by Abu Hurayrah, in which Allah’s Messenger (peace and blessings be upon him) said: “Whoever eats or drinks forgetfully should complete his fast, for what he has eaten or drunk has been granted to him by Allah.” (Reported by Al-Bukhari and Muslim) This hadith clearly shows that the one who unintentionally eats or drinks while fasting in Ramadan should complete his fasting, and he will not have to make up for that day later after Ramadan. This is because what such a person has eaten or drunk is considered to have been granted to him by Allah. Thus, eating or drinking forgetfully does not affect the validity of one’s fasting, because the one who forgetfully does something is pardoned for it, as stated in the hadith in which the Prophet (peace and blessings be upon him) said: “My nation are pardoned (by Allah) for whatever they do by mistake, whatever they do out of forgetfulness and whatever they are forced to do.” (Reported by At-Tabarani, Ad-Daraqutni and Al-Hakim but with different wording)

As for those who maintain that one should not remind whoever eats or drinks forgetfully in Ramadan on the pretext that it is something granted to him by Allah, it is an invalid opinion due to the following:
First: Eating or drinking during the daytime of any day in Ramadan is legally prohibited and is regarded as an evil deed. Therefore, whoever sees such an act should deny and forbid it, even if the one who does it is legally excused. So, reminding such a person of his being in a state of fasting is an act of enjoining what is good and forbidding what is evil, for eating or drinking in a daytime of Ramadan is considered to be an evil deed.
Second: Leaving the person who unintentionally eats or drinks while fasting in Ramadan to continue eating or drinking will make those who do not fast dare to publicly eat and drink during the daytime of the days of Ramadan.
Third: Reminding such a person that he is in a state of fasting is obligatory, for it is an act of helping each other unto righteousness and piety as commanded by Almighty Allah Who says: (… help ye one another unto righteousness and pious duty. Help not one another unto sin and transgression…) (Al-Ma’idah: 2)
Fourth: There is a kind of analogy between this case and that of reminding whoever forgetfully errs while performing Prayer. The one who is led in Prayer is enjoined to say tasbeeh (that is, Subhana Allah) loudly to attract the attention of the imam (the one leading the Prayer) that he has forgetfully erred during the Prayer. Ibn Mas`ud (may Allah be pleased with him) narrated: “The Prophet (peace and blessings be upon him) led us in Prayer, and when he had finished the Prayer he was asked, ‘O Allah’s Messenger! Has there been any change in the Prayers?’ He said, ‘What is it?’ The people said, ‘You have prayed so much and so much.’ So the Prophet bent his legs, faced the qiblah (Prayer direction; towards the Ka`bah) and performed two prostrations of forgetfulness and finished his Prayer with Tasleem (by saying: ‘As-Salamu `alaikum warahmat-ullah,’ that is, Allah’s peace and blessings be with you). When he turned his face to us he said, ‘If there had been anything changed in the Prayer, surely I would have informed you, but I am a human being like you and liable to forget like you. So if I forget, remind me and if anyone of you is doubtful about his Prayer, he should follow what he thinks to be correct and complete his Prayer accordingly and finish it and perform the two prostrations of forgetfulness.’” (Reported by Al-Bukhari and Muslim)
Among the anecdotes narrated in this connection is the following incident stated by Al-Hafizh Ibn Hajar Al-`Asqalani on the authority of `Abd Ar-Raziq, Ibn Juraij and `Amr ibn Dinar respectively: “A man came to Abu Hurayrah (may Allah be pleased with him) and said to him, ‘I was in a state of fasting, then I forgetfully ate something!’ Abu Hurayrah said, ‘It is all right.’ The man added, ‘Then I went to someone and I forgetfully ate and drank!’ Abu Hurayrah said, ‘It is all right. What you ate and drank was granted to you by Allah.’ The man continued, ‘Then I went to someone else and ate something!’ Abu Hurayrah said, ‘Then you are a man who is not used to perform fasting!’” (See Fathul-Bari, Bestowal of the Creator, Vol. 4) To conclude, it is obligatory to remind the one who forgetfully eats or drinks while fasting in Ramadan, and it is invalid to maintain that one should not remind whoever eats or drinks forgetfully in Ramadan on the pretext that it is something granted to him by Allah.