Eating or drinking out of forgetfulness does not invalidate the fast. In this regard, there is no distinction between obligatory (farḍ), necessary (wājib), and voluntary (nafl) fasts. The Prophet (peace and blessings be upon him) stated:
“Whoever forgets while fasting and eats or drinks should complete his fast, for it is Allah who has fed him and given him drink.”
(al-Bukhārī, ḥadīth no. 1933; Muslim, ḥadīth no. 1155)
When one encounters a fasting person who is eating forgetfully, the matter should be considered carefully. If the person appears strong enough to continue fasting, then—according to the preferred opinion—not reminding him that he is fasting is considered makrūh tahrīman (reprehensible to a degree close to prohibition). However, if the individual is very elderly or weak, then it is deemed more appropriate not to remind him. As indicated by the wording of the ḥadīth, this situation may be understood as a provision granted by Allah to that person out of mercy, causing him to forget due to his frailty.
If, however, a person who is eating while fasting is reminded—by being told, “You are fasting”—and he continues to eat without concern, then according to the sound opinion, his fast is invalidated and he must make it up (qaḍāʾ).