Is ending a romantic relationship for the sake of Allah considered an injustice to the young man who claims to have been harmed by this decision, and is there a fear that his prayer against the girl will be answered?

You have done well by ending the sinful relationship. The other party has no right over you, and you have not wronged him. He is not permitted to supplicate against you, for his supplication, being for a sin, prevents its acceptance. It is important to emphasize the impermissibility of any relationship between a man and a woman outside the framework of a valid marriage. If marriage is possible for those who love each other, it is better, while observing religion and character.