Your burden for this brother is heard and we join you in bringing him before the throne of God in the name of Jesus.
The first need for anyone who has drifted from the Lord through sexual sin is repentance. That word means a real change of mind and direction, a turning back to God. John the Baptist preached a baptism of repentance for the forgiveness of sins, and Jesus himself called sinners to repentance. When David sinned with Bathsheba and had Uriah killed, he confessed that his sin was ultimately against God. True faith is shown by that kind of turning, by a changed life and a restored walk with the Lord. Our prayer is that this brother would experience that work of God, that he would confess his sin honestly before God, turn from what is wrong, and be restored to fellowship with Christ. That restoration with God must come first, and from it all other restoration flows.
You also asked for prayer that he would accept God's will regarding an unequally yoked relationship and understand that a believer is not to be bound together with an unbeliever. Scripture is clear on this, be not unequally yoked together with unbelievers, for what fellowship has righteousness with unrighteousness and what communion has light with darkness. This principle is not limited to marriage, but it certainly includes marriage, and it applies to any close yoke where two are bound together and yet live by different standards. Many people think their situation is the exception, that love will overcome the difference or that the other person will change after the vows. Yet the warning remains because the yoke is always unequal. Two people who do not share the same Lord do not live by the same ethic and do not follow the same rules. What would grieve the conscience of a believer may not trouble an unbeliever at all, and that difference brings continual strain and temptation to compromise.
God's design for marriage is that the two become one, a deep unity. That is why the call to not be unequally yoked is an act of protection and love. Hosea shows the heart of God who betroths his people to himself forever in righteousness and justice, in lovingkindness and mercy, and in faithfulness, so that they may know the Lord. That is the kind of faithful, covenant love God desires for his people to reflect, and it can only be lived out when both husband and wife belong to him and seek to follow him together.
So we pray that God would give this brother unwavering faith in Jesus Christ and a clear understanding of the Father's holy plan for his life. We ask that the Lord would grant him a spirit of repentance, that he would hunger for righteousness and walk in purity, and that he would have wisdom and courage to order his household according to Scripture. Where there is adultery and unbelief, we pray that he would not make peace with sin but would seek what is holy and right before God, not out of harshness or vengeance toward any person, but out of obedience and love for Christ. We pray that he would receive godly counsel, that he would be surrounded by believers who encourage him to follow the Lord without persecution or pressure from others, and that every decision would be guided by the Word of God and the leading of the Holy Spirit.
May the Lord have mercy, bring conviction where needed, strengthen faith, and be glorified in this brother's life as he is drawn back to know the Lord more deeply and to walk in faithfulness before him. Amen.