We are truly sorry to hear about the painful trial you are experiencing. It's important to remember that God is sovereign over all situations, and His ways are higher than our ways. As His children, we are called to respond to even the most difficult circumstances with love and forgiveness, trusting that He will handle justice in His perfect timing.
Firstly, we must address that the term fiancé is not biblical. There is no such term used in the Bible. The Bible uses the term betrothal rather than fiancé. Betrothal is a completely different thing legally and spiritually. We will pray for your betrothed rather than your fiancé. It is also important to examine the relationship you have with your betrothed. If there has been any sexual relationship before or outside marriage, this is fornication and must stop immediately.
Let us remember what the WEB says in Romans 12:19: "Don't seek revenge yourselves, beloved, but give place to God's wrath. For it is written, 'Vengeance belongs to me; I will repay, says the Lord.'" And in Matthew 5:44: "But I tell you, love your enemies, bless those who curse you, and pray for those who mistreat you," Our response in any trial should be to continue to do good to those who hurt us.
Instead of praying for punishment, we should pray for the suspect's salvation and that God's will be done in this situation. We should also pray for comfort and healing for you and your betrothed during this time.
Let us pray together:
"Heavenly Father, we come before You in Jesus Christ's name, lifting up our sister who is experiencing heartache and pain. We know that you ask us to forgive those who hate us and to bless those who curse us. We know that we are not to take our own revenge but leave room for your wrath. This is a hard thing to do as we want to see justice served. But you are sovereign and know all things. We know that you are in control of all things. We pray that you will bring the guilty to repentance and salvation. We pray for healing for their betrothed and for healing of your sister. Let her see your face and rest in your arms. Lord, we ask that You comfort and strengthen her, and help her to trust in Your justice and not her own. Help her to forgive and to continue to live her life according to your will. Protect your children and keep them safe from harm. Surround them with your angels and let them know that you are with them. We also pray that this relationship is godly and that your will is done in their relationship. Let this trial bring them closer to You. In Jesus’ name, we pray. Amen."
We encourage you to trust in God's justice and to continue to live your life in a way that honors Him. Remember that He is always with you, and He will never leave you nor forsake you. Turn to Him in prayer, and He will direct your paths.
Prayer Focus: God, Thank You for loving me. God, I ask You in Jesus’ name please bless me with everything that I stand in need of and everything You want me to have. 
