We are truly saddened to hear about the situation your daughter is in. Let's bring this matter to the Lord in prayer and seek His will according to His word.
Firstly, it is important to address that God's design for a relationship is that it be between a man and a woman who are both believers and that it should be moving toward marriage. The Bible tells us not to be unequally yoked with unbelievers (2 Corinthians 6:14 WEB), and that marriage is to be held in honor among all, and the marriage bed be kept undefiled (Hebrews 13:4 WEB).
Your daughter's relationship with a man who has a history of multiple affairs and a recent divorce is a concern. The Bible is clear about God's view on divorce and adultery. In Malachi 2:16 WEB, it says, "For I hate divorce, says Yahweh, the God of Israel."
It is also clear that sex outside of marriage is fornication and not pleasing to God. We are not to allow sin to reign in our mortal body, that we should obey it in its lusts. Neither present your members to sin as instruments of unrighteousness, but present yourselves to God, as alive from the dead, and your members as instruments of righteousness to God. (Romans 6:12-13)
We need to pray that God will open your daughter's eyes to see the truth about this relationship. We pray that God will bring a godly man into her life who fears Him and will love her as Christ loves the church.
Let's pray:
Dear Heavenly Father,
We come before You today, lifting up our daughter to You. Lord, we ask that You open her eyes and her heart to see the truth about this relationship. Help her to see that this man's past actions and current situation do not align with Your will for her life. Lord, we ask that You bring a godly man into her life who fears You and will love her as You intend. We pray and hope that You will also work in this other's man heart to convict him of his actions that have hurt others, including our daughter, and to seek You.
We also pray that our daughter will seek You with her whole heart and learn about You and Your plans for her life. We pray that she will accept Jesus Christ as her savior if she has not already. We pray that You will condemn the hearts of our daughter and this man and guide them both to repent of their sins and turn to You. Help them to seek their relationship with You above all else.
In Jesus' name, we pray. Amen.
We encourage you to continue to guide your daughter in the way of the Lord and the word of God. Show her the truth of Jesus Christ and continue to pray for her. If Jesus Christ is not mentioned in your prayers, we encourage you to remember that it is only by the name of Jesus that we have access to God. As it says in John 14:6, "Jesus said to him, I am the way, the truth, and the life. No one comes to the Father, except through me." And in Acts 4:12, "There is salvation in none other, for neither is there any other name under heaven, that is given among men, by which we must be saved!"
Prayer Focus: God, Thank You. Thank You for loving me. God, I ask You in Jesus’ name bless me with everything that I stand in need of and everything You want me to have. Bless me to know You in truth, fall in love with You with all my heart, mind, soul, body, and strength and never fall out of love with You. Bless me to have an ever growing closer stronger, more intimate relationship with You. Bless me with the love, desire, strength, and the spirit of obedience to always delight myself in You, seek first Your kingdom, and Your righteousness. Help and strengthen me God to always respect and obey You. Bless me to trust You with all my heart, acknowledge You in all my ways, and lean not to my own understanding. 
