If menstruation ends at noon, the woman follows the remaining-day abstention rule described in the source.
Since menstruation existed at dawn, that day was not a valid obligatory fast from its beginning. She must also make up the day later; abstention from noon does not retroactively turn it into a complete fast.
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Kitab al-Sawm: Unconsciousness, Recovery, Menstruation and Abstention
If menstruation ends at noon, the woman follows the remaining-day abstention rule described in the source.
Since menstruation existed at dawn, that day was not a valid obligatory fast from its beginning. She must also make up the day later; abstention from noon does not retroactively turn it into a complete fast.