Is it permissible for a person who was instrumental in the guidance of a repentant female fornicator to insult her, curse her, and remind her of her past due to unintentional mistakes on her part, such as not lowering her gaze, knowing that she quickly repents from them?

It is not permissible for a husband to insult his wife or taunt her with a sin from which she has repented, especially since Allah has graced her with guidance. The husband should be reminded that his action is reprehensible, both religiously and naturally. The fact that he was a cause for her guidance does not change the ruling; rather, it makes his taunting uglier. He should be advised to cease harming her, thereby completing his righteous deed.