I say, and with God's success: She is not required to perform ghusl; since at the time of cessation she was a disbeliever, and she is not commanded with the laws in our view. Whenever she converts to Islam, the reason, which is the cessation, no longer exists, and the cessation is not continuous, unlike if she had been in a state of major impurity while being a disbeliever, then converted to Islam; in that case, she must perform the ghusl for major impurity because major impurity is a continuous matter, and she would be in a state of major impurity after converting to Islam. See: Sharh al-Wiqayah, p. 95, and God knows best.