Excerpts from the ‘Jurisprudence of Worship’
By Shaykh Muhammad ibn Saalih ibn Uthaymeen رحمه الله تعالى

Question: Oh Noble Shaykh, It has become widespread in many people that if one sees a fasting person eating that he should not remind him (that he is fasting), to what extent is this statement correct?

Answer:
Whomever sees a fasting person should remind him (that he is fasting). This is because it is an example of aiding one another in virtue, righteous and piety. Similar to the case of one who sees a person praying in a direction away from the Qiblah (the direction to the House of Allāh) or in the case of one who sees a person that is about to make Wahoo (ritual purification) with water that is impure or what is similar to this. Therefore, in all these cases it is obligatory to clarify the matter to the person. While, the fasting person is excused for his forgetfulness, his brother, who knows his situation, is obligated to remind him. Perhaps this can be addressed as well with the statement of the Prophet ﷺ:

“But I am only a human being like you and liable to forget like you. So if I forget remind me.”¹

So verily if the forgetful one is reminded in Salāh (prayer), similarly the forgetful one is reminded of his fast.
Jurisprudence of Worship- Fatwa Regarding Sáum (Fasting). By Shaykh Muhammad bin Saalih ibn Uthaymeen (pg. 277)

¹Related by Bukhari – The Book of Salāh (Prayer) – Number [401] and related by Muslim – The Book of Mosques and Places of Prayer – Number [572].

Translated by: Abu Uwāis, Muhammad Abdullahi Elmi