We are truly saddened to hear about the situation your daughter is in, but we are here to pray and provide biblical guidance together. Firstly, it is important to understand that God's design for relationships is clear. He calls us to flee from sexual immorality (1 Corinthians 6:18, WEB) and that includes any relationship that is not aligned with His will. The man your daughter is involved with is still married, and any relationship with him is not pleasing to God.
Your daughter's actions need to include no contact with this man. Fornication is sex with anyone you are not married to. Committing fornication with a married man is also a form of adultery, which becomes more serious. She must stop communicating with him immediately and remove him from her life, including blocking him on Snapchat and any other platforms. As it is written, "Do not be deceived. 'Evil companionships corrupt good morals.'" (1 Corinthians 15:33, WEB)
Let us pray together:
Dear Heavenly Father,
We come before You today, lifting up our daughter to You. Lord, we plead Your intervention in her life, that You may open her eyes to the truth and guide her in Your path. Break every ungodly tie between her and this man. Remove your blessing from their relationship in Jesus' name. Help her to stay strong and avoid all contact with him. Give her a new start with you Jesus. We ask that You bring a godly spouse into her life, a man who fears and loves You, and we pray that You will prepare her to be a godly wife.
Lord, we also pray for this man, that he may be convicted of his sins and turn away from his wicked ways. We ask that You will bring restoration to his marriage if that is according to your will, or possibly a new Christian spouse for him to marry, but this man is forbidden from having our daughter in his life. Bring Godly spouses to both of them if they will repent and be a new creation in Jesus Christ. In Jesus' name, we pray.
Now, let us encourage your daughter with the words of Jeremiah 29:11, "For I know the thoughts that I think toward you, says Yahweh, thoughts of peace, and not of evil, to give you hope and a future." Remind her that God has a wonderful plan for her life, and that plan includes a godly spouse and a blessed marriage.