Has she always been atheist/agnostic? I forget whether you mentioned that or not.
For the record, you one hangs out with has really no bearing on that individual. Don't believe me; take a look at those who walked CLOSELY with Jesus.
I'll holla!
No, she used to be saved, sanctified, and Holy Ghost-filled. But, I didn't know her then. When I met her, she considered herself an atheist. After a few conversations, I determined that she's more agnostic than atheist.
And as for that second paragraph: COSIGN.

So you.... being in the body of Christ... would... as a mature spiritual individual ... apply GAL 6:1 "Brethren, if a man be overtaken in a fault, ye which are spiritual, restore such an one in the spirit of meekness; considering thyself, lest thou also be tempted."
So is it safe to say... that out of love for the body of Christ... you would just remind me...even though I fell and know better, that what im doing is wrong and that God is has extended mercy to the believers and that I should repent and seek him for delieverence so that I can be victorious in my dealings with the spirit of lust.
I live, breathe, and eat Galatians 6:1. I think it's my most-quoted verse. So yeah, that's pretty much the approach I'd take. I believe that if you are saved, then you already know you shouldn't be having pre-marital sex. All you need from me is love, prayer, and encouragement. If you intend to continue having sex with this dude, I'm not going to continue to preach to you. I'll share with you one time, and leave you in God's hands.