He must refrain for the rest of the day in imitation of the fasters, as well as anyone who has a valid excuse preventing the obligation of fasting or permitting breaking the fast at the beginning of the day, and then their excuse is removed, and they are in a state where if they had been in that state at the beginning of the day, they would have been required to fast. It is not permissible for them to break their fast; they must refrain for the rest of the day in imitation of the fasters: such as a boy who reaches puberty during the day, a non-Muslim who converts to Islam, a mentally incapacitated person who regains sanity, and a menstruating woman who becomes pure. They are required to refrain for the rest of the day in imitation of the fasters. See: Durar al-Hukam 1: 204-205, Radd al-Muhtar 1: 253, and Bada'i al-Sana'i 2: 103; it was narrated from Salamah ibn al-Akwa' (may Allah be pleased with him) that the Prophet (peace be upon him) commanded a man from the Ansar to announce to the people that whoever has eaten should fast the rest of his day, and whoever has not eaten should fast, for today is the day of Ashura. This is found in Sahih al-Bukhari 2: 705, Sahih Ibn Hibban 8: 385, and Al-Mustadrak 3: 608, and Ashura was obligatory to fast before the obligation of Ramadan. And because the time of Ramadan is a noble time, it is necessary to honor this time as much as possible. If one is unable to honor it by actually fasting, they must honor it by imitating the fasters; fulfilling its right as much as possible if they are capable of imitation, and to avoid exposing themselves to suspicion. See: Bada'i al-Sana'i 2: 103.