I say, and with God's success: If she becomes pure during the day and has not eaten anything, the fast of this day is not valid, but she must refrain from eating, due to the sanctity of Ramadan, just as it is required of a traveler who stays after midday, a madman who regains his sanity, a sick person who recovers, a boy who reaches maturity, and a disbeliever who converts to Islam. Refer to: Umdat al-Ri'ayah 1: 129, and God knows best.