What is the ruling of Islam regarding the completion of a marriage to a Muslim girl whose fiancé discovered that she had lost her virginity due to a previous relationship, given that they have already announced their engagement, and she claims repentance?

A servant (of God) must conceal their own sins if they commit a transgression, and spying is not permissible unless it is to prevent a manifest evil. The girl erred by informing you of her forbidden relationship, and you erred by spying on her. If the girl has repented, there is no impediment to marrying her, because repentance erases what came before it. However, if you are suspicious about her matter, it is advisable to leave her and seek someone else who possesses religion and good character. An engagement is a promise of marriage, and it is disliked to break it unless there is a benefit in doing so, and a suitor is a stranger to his fiancée before the marriage contract.