We are truly saddened to hear about the situation your daughter-in-law is in, and we are here to pray and provide biblical guidance with you. Firstly, it is important to acknowledge that the relationship your daughter-in-law is engaged in is adultery, which is clearly forbidden in the Bible. In Exodus 20:14 WEB, it is stated, "You shall not commit adultery." This sin has not only affected her, but also her children and their father, causing immense pain and distress.
We must also address the fact that the man she is involved with is also committing adultery, as he has his own family. This situation is a clear example of the devastation that infidelity brings. In Proverbs 6:32 WEB, it says, "But whoever commits adultery with a woman lacks understanding. He who does it destroys his own soul."
Let us pray together for this situation:
Dear Heavenly Father,
We come before You today, lifting up this family in desperate need of Your intervention. We pray for Your mercy and grace to be poured out upon them. Lord, we ask that You convict the hearts of both your daughter-in-law and the man she is involved with, that they may see the error of their ways and turn away from this sin. We pray, that you remove this man from her life.
Father, we also pray for the healing and restoration of her marriage. You have said that You hate divorce which means it is against Your will. We pray that You would help her family to forgive and heal and that they would be able to rebuild their marriage and that the love would be restored stronger than before. Malachi 2:16 WEB, states "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."
We ask this in the precious name of Jesus Christ, who died on the cross for our sins and rose again for our justification. It is only through His name that we have access to You, Father, and it is only through His name that we can be saved. If you claim to be a Christian, we ask that in every aspect that you would have faith in Him and seek His will. In James 4:3 WEB, it says, "You ask and don't receive, because you ask with wrong motives, so that you may spend it for your pleasures." Therefore, we pray that your daughter-in-law's heart would desire the Father, and that her faith would be in Jesus, and not in the things of this world.
We also pray for the father of the children, that You would give him the strength and wisdom to support his family through this difficult time, and You would heal his broken heart. We pray that You surround the children with Your loving presence, that You may protect them and comfort them and draw them to Yourself.
In Jesus' name, we pray. Amen.
May God strengthen you and give you divine direction to act accordingly.
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. 
