Eating forgetfull during fasting

Question :

What is the ruling on eating or drinking forgetfully? Is it obligatory upon whoever sees the person eating or drinking forgetfully to remind him that he is fasting?

Answer:

Whoever eats or drinks forgetfully while he is fasting, then verily his fast is correct. However, when he remembers he must stop, even if a morsel of food or some drink is in his mouth he must spit it out. The proof of the completion of his fast is the Prophet's statement in what has been confirmed from him by way of Abu Hurairah :
"Whoever forgets while he is fasting and he eats or drinks, then let him complete his fast. For verily, it was only Allah who fed him and gave him drink. "

This is also because a person is not punished for forgetfulness in doing a prohibited action. This is due to the Statement of Allâh, the Most High:
Our Lord, do not punish us if we forget or fall into error."

" Allah, the Most High, said: "I have done so (i.e., pardoned it)."

In reference to whoever sees him, it is obligatory upon him to remind him, because this is a part of changing an evil. Verily, the Prophet said:
"Whoever among you sees an evil, then let him change it with his hand, and if he is not able, then with his tongue, and if he is not able, then with his heart."

There is no doubt that the fasting person's eating and drinking during his fast is a form of evil, but he is pardoned for it while he does so forgetfully, as there is no punishment for it. However, concerning whoever sees him, he has no excuse for avoiding stopping him.

Source:
Ash-Shaykh Ibn 'Uthaimin
Fatawa Islamiyah, Vol. 3 Pages