We are truly saddened to hear about the struggle your daughter's father is facing, but we are encouraged by your faith in God's power to save and heal. Let us come together in prayer, seeking God's will for his life.
Firstly, we acknowledge that the salvation of God in Jesus Christ is the only way to heal and save. It is only in the name of Jesus Christ that any of this is possible as Jesus is the waymaker and miracle worker. If you have not done so, please talk to your daughter's father and encourage him to believe in Jesus Christ.
Help him to understand that the only way to God is through Jesus Christ. For more information and help with discussing this, look at scripture such as John 3:16, John 14:6-7, and Romans 10:9-10.
In your prayer, you mentioned that he is your best friend. We are assuming that you are not married to this man and are perhaps in a relationship with him of some sort. The Bible does warn us against being unequally yoked with unbelievers.
Do not be unequally yoked with unbelievers, for what fellowship have righteousness and iniquity? Or what fellowship has light with darkness? What agreement has Christ with Belial? Or what portion has a believer with an unbeliever?
2 Corinthians 6:14-15 WEB
Also, if you are in a romantic relationship with this man, then you both should be considering marriage. It is clear in the Bible that a man and a woman should not be in a close relationship unless they are married or getting married soon. If you are not married or going to be married soon, then you should be aware that God wants us to abstain from fornication.
The body is not for sexual immorality, but for the Lord, and the Lord for the body.
1 Corinthians 6:13 WEB
We are strongly urging you to consider marriage if you are in a romantic relationship with the father of your daughter.
Regarding his addiction to drugs, Jesus himself addressed the heartache and bondage of addiction.
The Spirit of the Lord is on me, because he has anointed me to preach good news to the poor. He has sent me to heal the brokenhearted, to proclaim release to the captives, recovering of sight to the blind, to deliver those who are crushed,
Luke 4:18 WEB
Therefore, let us pray together in Jesus' name.
Dear Heavenly Father,
We come before you, lifting up the father of this daughter to you. We pray that he turns his heart to you and believes in Jesus Christ. We ask that you deliver him from his addiction, that you break the chains of bondage, and set him free indeed as your word promises. We pray that you wrap your loving arms around him, and give him the strength to seek help and go to rehab. We ask that you keep him safe and alive, and that you heal his spirit. We pray that he finds the strength and guidance needed for a lasting recovery. We pray that your Holy Spirit gives him the power to overcome the addiction and to glorify you in the process. We also ask that you give him the wisdom to see that God has a better way and to follow that way. Let him be a light to others who may be in a similar situation. Finally, we pray that you guide the relationship between the two of them and that your will is followed in their lives regardless of the situation.
We ask this in Jesus' name. Amen.
Be encouraged, for God is indeed the waymaker and miracle worker. He can turn any situation around for his glory. Trust in him, and continue to seek his will in your lives. Also, remember that God is not only the Father of your daughter but also is the father of all of humanity. He is a personal God and wants a personal relationship with all of us. He is a father to the fatherless and a husband to the widowed. Psalm 68:5. God wants to be your daughter's father and your husband. Just as he wants to be a father to the father of your daughter and a husband to you.
Consider the following verse that offers hope and encouragement.
Trust in Yahweh with all your heart, and do not lean on your own understanding. In all your ways acknowledge him, and he will make your paths straight.
Proverbs 3:5-6 WEB
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. 
