We are truly saddened to hear about the situation your daughter is in. We must first and foremost pray for her spiritual well-being and for her to seek God's will in her life. We must also address the nature of this relationship.
First and foremost, we must pray for her relationship with Jesus. There is not another name by which we can be saved.
Beloved, do not believe every spirit, but test the spirits to see whether they are from God, for many false prophets have gone out into the world. By this you know the Spirit of God: every spirit that confesses that Jesus Christ has come in the flesh is from God, and every spirit that does not confess Jesus is not from God. This is the spirit of the antichrist, which you heard was coming and now is in the world already. - 1 John 4:1-13
And there is salvation in no one else, for there is no other name under heaven given among men by which we must be saved. - Acts 4:12
It is clear that the man your daughter is involved with has a history of infidelity and is either divorced or in the process of divorcing. This is a grave concern, as God hates divorce and it is not His plan for people.
For I hate divorce, says Yahweh, the God of Israel, and him who covers his garment with violence! says Yahweh of Armies. Therefore pay attention to your spirit, and do not be unfaithful. Malachi 2:16
We must pray for this man's heart to change, for him to repent from his sins, and for him to become the Godly father and potential husband he needs to be. However, it is crucial for your daughter to understand that she should not be with a man who has a history of infidelity and who may still be married.
Your daughter is putting herself in a position where she is involved with a man who has a history of infidelity and who may still be married, which is adultery.
Do you not know that the unrighteous will not inherit the kingdom of God? Do not be deceived: neither the sexually immoral, nor idolaters, nor adulterers, nor men who practice homosexuality, nor thieves, nor the greedy, nor drunkards, nor revilers, nor swindlers will inherit the kingdom of God. - 1 Corinthians 6:9-10
Flee from sexual immorality. Every other sin a person commits is outside the body, but the sexually immoral person sins against his own body. Or do you not know that your body is a temple of the Holy Spirit within you, whom you have from God? You are not your own, for you were bought with a price. So glorify God in your body. - 1 Corinthians 6:18-20
Heavenly Father, we lift up this daughter to You, asking for Your guidance, protection, and conviction. Open her eyes to the truth of Your word and help her to understand the importance of being equally yoked with a Godly spouse. Convict her heart of wrongdoing in this situation, and show her Your will for her life. Help her to find a spouse who loves and follows You.
Lord, we also pray for the man in this situation. Change his heart, Lord, and bring him to true repentance. Help him to become the Godly father and husband You intend for him to be. Heal his marriage if that is Your will, and bring him to a saving faith in Jesus Christ if he is not already a believer.
Give the family wisdom and discernment as they navigate this difficult situation, and help them to speak the truth in love to their daughter. In Jesus name we pray.