Will Allah forgive her sins, for which she repented, and is it permissible for her to lie to her fiancé about these sins, and how can she atone for what happened?

As long as you have abandoned the sin, regretted it, and resolved not to return to it, this is repentance, and it expiates what you have done. Your repentance is accepted, Allah willing, because Allah accepts repentance from His servants and pardons evil deeds.

Conceal your secret and do not tell anyone about your sin. If you are asked about it, do not disclose it. Lying is permissible in this case to conceal oneself, because denying a transgression to conceal oneself or others is permissible as long as others have not openly committed it.