We understand your concern and are here to pray and counsel with you, dear friend. Firstly, let's remember what the Word of God says about vengeance: "Avenge not yourselves, beloved, but give place unto the wrath of God: for it is written, 'Vengeance belongeth unto me; I will recompense,' says the Lord." (Romans 12:19, WEB)
It's important to note that the battle you're facing is not against flesh and blood, but against spiritual forces. "For our wrestling is not against flesh and blood, but against the principalities, against the powers, against the world's rulers of the darkness of this age, and against the spiritual forces of wickedness in the heavenly places." (Ephesians 6:12, WEB)
Let's pray together:
"Dear Heavenly Father, we come before You in Jesus' name, lifting up our brother/sister who is facing this trial. Lord, You know the situation and the hearts of all involved. We ask that You intervene and remove any influence of the enemy that seeks to destroy this relationship. Please give our sibling in Christ the strength and wisdom to handle this situation in a way that honors You. Help them to forgive, to love, and to trust in Your timing and Your ways. Expose any wickedness and bring it to light, Lord. In Jesus' name, we pray. Amen."
We also encourage you to forgive and bless this lady, as Jesus commanded us to love our enemies and pray for those who persecute us (Matthew 5:44, WEB). Remember that God is faithful and just. He will fight your battles and make a way where there seems to be no way (Isaiah 43:19, WEB).
Lastly, we want to ensure that your relationship with your fiancé is built on a strong foundation of faith in Jesus Christ. If you haven't already, we encourage you both to commit your lives and your relationship to Him. It's only through Jesus that we have access to God, and there is no other name by which we can be saved (Acts 4:12, WEB).