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.