What should a fiancé do with his fiancée who agreed to wear the niqab after marriage but then changed her mind, knowing that she has memorized half of the Qur'an and he is committed to her?

Regarding the permissibility of a woman uncovering her face in front of non-mahram men, a wife must obey her husband if he orders her to cover her face. If you believe that covering the face is obligatory, then instruct your wife to do so, and explain to her that a Muslim is commanded to adhere to the rulings of the Sharia. However, if you believe that uncovering the face is permissible, then there is no objection to affirming her in that and abiding by her condition if she stipulated not being compelled to cover. This is alongside encouraging her towards the niqab, informing her of its virtue, assisting her in learning the rulings of theia, and strengthening her connection with Allah. It must be noted that a fiancé is considered a non-mahram to his fiancée as long as their marriage contract has not been concluded.