Is abstaining from sin worth anything if it is done for the wrong reasons? For example, if a person abstains from sin because they don't want to disappoint people or saints instead of abstaining out of love for God, is the abstinence from sin in such a case a sin in itself?
To give a more particular example: let's say that an agnostic girl who used to live a depraved life (engaged in promiscuity, fornication, drugs etc.) falls in love with a Christian guy and he reciprocates her love, but he tells her that they can only get married if she strives to repent and stop sinning. She doesn't believe in God, but because she is very much in love with the Christian guy, she quits all her previous sinful habits. She starts living a life that could be considered righteous (she dresses modestly, changes her behavior and quits all drugs and all forms of fornication), she attends church and even tries to pray, but she only does so in order to please her partner, without believing that God actually exists. She is honest to her boyfriend about her lack of belief and about the fact that she has turned over a new leaf out of love for him, not out of sincere Christian repentance. Should he marry her? Would her change of lifestyle amount to anything in God's eyes, or will she still be damned at the end of her days?
Answers from Christians belonging to all denominations are welcomed.
I'll answer you with Jesus' words:
"unless your righteousness SURPASSES that of the Pharisees and the teachers of the law, you will certainly not enter the kingdom of heaven." (Matthew 5:20)
She MUST be Born Again. Your girl is exactly like the people under John the Baptist's teaching when they obeyed, repented, and were baptized in water, except they believed in God, and even all that did not save them.....only Jesus can give the ONE Baptism of God's Holy Spirit. Jesus said "You MUST be Born Again" (John 3:7). And He said "Unless a man is Born Again, he cannot see the Kingdom of Heaven."
So NO. All those changes mean nothing, unless she repents AWAY from ALL sin TOWARD Jesus Christ and receives the Holy Spirit of God. As you described her, she has never repented of the Greatest Sin, which is to not Love the Lord your God with all your Heart, Mind, Soul, and Strength. This was also the final sin of the unrepentant "rich young ruler" (Mark 10:17-27) that kept him from being saved, and this Greatest Command can only be obeyed by someone who is Born of God.
The Greatest command is "Love the Lord your God with all your heart,mind,soul, and strength. And it is written "Whoever breaks one of the commands is guilty of them all". You cannot Love God with EVERYTHING in you, unless you are Born of God by the Spirit of Jesus Christ.
Unless the girl becomes Born of God, the boy CANNOT have Spiritual Unity with her because they are of 2 contrary spirits, and therefore, they should not be married. It's not good to "yoke" two different animals to a single plow. They just won't work well together.