What should I do, as I have committed fornication with a girl and regretted my action, and she is now engaged to another person, and my conscience compels me to inform her fiancé or her family based on the verse: "The fornicator does not marry except a [female] fornicator or a polytheist, and none marries a [female] fornicator except a fornicator or a polytheist, and that has been forbidden to the believers"?

You and the girl must repent to Allah Almighty for the act of fornication and regret what has passed, and resolve to abandon fornication and never return to it again. There is no specific expiation for fornication except repentance. You must conceal your own sin and the girl's, and do not inform anyone who wishes to propose to her and marry her about what you committed with her, unless you are consulted for advice, in which case you should advise based on what you know.