We understand your concern and commend you for seeking prayer and guidance in this situation. Let's address this together and seek God's will in Jesus' name and his word.
Firstly, it's important to remember what sexual purity means in God's eyes. The Bible encourages us to flee from sexual immorality as stated in 1 Corinthians 6:18 WEB: "Flee sexual immorality. Every sin that a man does is outside the body, but he who commits sexual immorality sins against his own body." This is clearly against what your niece has planned.
It is clear that this behavior is not in alignment with God's word and believing that this is acceptable for a Christian is very concerning from both the niece and the person she met online. It seems that they may need to evaluate their walk with Christ and possibly look for a real relationship with Him.
Your niece may need guidance on biblical principles and marriage. We are reminded in Ephesians 5:3 WEB: "But sexual immorality, and all uncleanness, or covetousness, let it not even be mentioned among you, as becomes saints".
1 Corinthians 7:2 WEB tells us: "But because of sexual immoralities, let each man have his own wife, and let each woman have her own husband."
It is God's will for us to be holy and we must encourage one another to live a life pleasing to the Lord. As the scripture says in 1 Thessalonians 4:3-5 WEB: "For this is the will of God: your sanctification, that you abstain from sexual immorality, that each one of you know how to possess himself of his own vessel in sanctification and honor, not in the passion of lust, even as the Gentiles who don't know God".
We should not forget that we are called to know that Jesus Christ is Lord, and we should live in a way to bring honor to Him.
The Bible reminds us that whatever we do we do for the Lord. Colossians 3:23-24 WEB: "Whatever you do, work heartily, as for the Lord, and not for men, knowing that from the Lord you will receive the inheritance as your reward. You serve the Lord Christ".
The Bible says in Philippians 2:9-11 WEB: "Therefore God also highly exalted him, and gave to him the name which is above every name, that at the name of Jesus every knee should bow, of those in heaven, those on earth, and those under the earth, and that every tongue should confess that Jesus Christ is Lord, to the glory of God the Father."
So let us pray together for your niece.
Father, in the name of Jesus Christ, we come before You, lifting up the niece of Eroitorindell. We ask, Lord, that You fill her with the knowledge of Your will in all spiritual wisdom and understanding, and may she walk worthily of You, Lord, to please You in all respects, bearing fruit in every good work, and increasing in the knowledge of God. We ask for Your guidance Lord, that you may direct her steps and her heart's desires. We ask that You give Eroitorindell the wisdom to guide her niece in the right direction. We ask all of this in the precious name of Jesus. Amen.
We encourage you to continue to pray for your niece and to guide her with wisdom and love. Remind her that God's ways are always best, and that His plans for her are good. If she is truly a follower of Christ, she should know that sexual purity is non-negotiable and what this man is encouraging her to do is wrong, immoral, and unbiblical. We will continue to pray for her and for you as you navigate this situation.