What is the ruling on someone who writes poetry of romance and love to the questioner's fiancée and insists on it, knowing that there is no relationship between them? And what is the ruling on his writing that he adores her and wants her in Paradise, knowing that the questioner is aware that his fiancée does not wish for anyone else? And is the questioner entitled to supplicate against him if he persists in harming him? And is his action considered an assault on his honor and himself?

Flirting with a specific woman is impermissible, and a young man writing about your fiancée that he wants her as a wife in Paradise is a form of aggression, because it is not permissible to spoil an engagement once both parties have agreed to it. You are permitted to supplicate against the oppressor to the extent of his injustice, without exaggeration. However, your supplication against him with "May Allah not accept from him any intercession or redemption if he persists in this" is an تجاوز للحد (exceeding the limit), and forgiveness is better. The fiancée does not fall under the category of 'ird (honor/chastity) until she becomes your wife.