She vowed to memorize Soorat Yoosuf and she did memorize it, but now she has forgotten some of the verses
The one who vows to memorize a soorah of the Qur’aan and does memorize it, has fulfilled his vow, even if he makes a mistake or forgets some parts of it after that. These mistakes and forgetting happen to most of those who memorize Qur’aan, but they can be corrected by means of regularly reviewing what one has memorized.
We advise you to keep refreshing your knowledge of the Qur’aan, and review it regularly. The Prophet (peace and blessings of Allaah be upon him) said: “Keep refreshing your knowledge of the Qur’aan, for by the One in Whose hand is the soul of Muhammad, it is more inclined to escape than a camel from its rope.” Narrated by al-Bukhaari (5033) and Muslim (791), who said.
What is meant is that if the Qur’aan is not reviewed regularly, it will escape from you, as the camel escapes from its rope with which it is tied.
We ask Allaah to help and guide us and you.
And Allaah knows best.