This is a story among many that I have heard of people who fell into sin, and believed that in the process they lost their salvation. This way of thinking is contrary to the gospel. To think that when we sin, we lose our salvation, is to think that we are saved by our own works. This is also to doubt the faithfulness of God, to think his love is as fickle as our own.
I cannot say that I know the heart of that girl to tell you whether or not she was saved before she began sleeping around, and before she repented, but I can say this: Christians are not saved by our works, but by our faith, and even that faith is a gift from God. When we are saved, God will not leave us, because he is faithful to his promise even when we are faithless in our obedience to him. God never chose to save us because we were good people, and he does not continue loving us because of our goodness. It was never about our goodness.
But Paul doesn't end there, he says, " Wretched man that I am! Who will deliver me from this body of death? Thanks be to God through Jesus Christ our Lord!" (Romans 7:24-25). As believers, Christ's righteousness has been imputed onto us so that we are no longer seen as guilty before God. During this life we will fall into sin, but if we cry out to God recognizing we are helpless, he will deliver us. For "The sacrifices of God are a broken spirit. A broken and contrite spirit, O Lord, you will not despise” (Psalm 51:17). God is our only hope, our only salvation.