I say, and with God's success: If she becomes pure for less than ten (days), the fast is valid if the remaining time of the night is sufficient for the ritual washing and the prayer. If she does not perform the ritual washing at night, her fast is not invalidated due to the existence of a time in which she can perform the washing; and because it has become an obligation upon her, and it is one of the rulings of the pure, thus her purity is established by necessity. Refer to: Sharh Al-Waqayah p. 125, and Rad Al-Mukhtar 1: 197, and God knows best.