We understand that you're seeking prayer for a loving and faithful relationship. However, we need to address a few things according to biblical principles.
Firstly, we notice that you haven't mentioned the name of Jesus Christ. It's important to remember that it is only through Jesus that we have access to God. As it says in John 14:6 WEB: "Jesus said to him, 'I am the way, the truth, and the life. No one comes to the Father except through me.'" And in Acts 4:12 WEB: "There is salvation in none other, for neither is there any other name under heaven, that is given among men, by which we must be saved."
Secondly, we need to clarify what you mean. Are you both in a courtship with the intention of a godly marriage? If not, we encourage you to consider that God's design for relationships is for a man and a woman to be joined in holy matrimony, a lifelong commitment that mirrors Christ's love for the church.
In 1 Corinthians 7:2 WEB states to avoid sexual immorality. If you are in a relationship outside of marriage that is not a courtship then you are in sin. 1 Corinthians 6:9-10 WEB states "Or don’t you know that the unrighteous will not inherit the Kingdom of God? Don’t be deceived. Neither the sexually immoral, nor idolaters, nor adulterers, nor male prostitutes, nor homosexuals, nor thieves, nor covetous, nor drunkards, nor slanderers, nor extortioners, will inherit the Kingdom of God."
Finally, regarding your concern about faithfulness, 1 Corinthians 13:7 says WEB Love bears all things, believes all things, hopes all things, and endures all things. The passage goes on to say that love is not selfish and it does not behave rudely. This is the kind of love that leads to faithfulness.
Let us pray for you:
Heavenly Father, we come before you in the name of Jesus Christ. If this person and their other are in a relationship that is not in line with your word, then we pray for them to come to a mutual repentance and a mutual acceptance of the basic Christian message of salvation. If it is a courtship, that they will trust in you and follow your principles for relationships. We ask that you help them to love each other with a pure, selfless love that bears all things and endures all things. Help them to be faithful to each other, and to you, Lord. Draw them closer to you, Jesus, and guide them in your will for their lives. May their relationship bring glory to your name. In Jesus' name, we pray. Amen.