First of all, I applaud your desire to live a life of purity yourself and secondly, your desire to encourage your friend to live a life of purity. It isn't easy, in this culture to go against the tide of impurity. And it seems your friend, like so many others, has bought into those lies.
I think the reason that people buy into the lies is because they are sincerely seeking something in their life. They are looking for love, for security, for meaning . . . they just are being misguided by the world.
The best thing you can do for your friend is this:
1) Love her and be the best friend that you can be for her. Let her know the sincerity of your friendship!
2) Continue to live a life of purity yourself. Your living example speaks volumes. Even if she makes mistakes now -- what she sees you doing in your life is having an effect on her. That might be the very thing that turns her life around later on. Hopefully though, she won't make those mistakes.
How can you be of immediate help to her?. . .
3) Pray and offer up sacrifices for her purity!!! The power of prayer and penance can not be underestimated!!! Sacrifices bring about tremendous grace -- it is the most powerful sign of love -- just look at the Cross!!!
4) Continue to talk to her about purity as she allows you to. Don't be pushy. Always come at it from the perspective of how purity sets you free to be the best you can be and to get the most out of life that God intends for you! Don't come down on her from a negative perspective that will lead her to discouragement. Our God forgives freely. We don't want people to ever feel that once they have "crossed that line" or fallen into a sinful lifestyle that they can never come back! Ideally, we don't want her to give away the gift of her virginity to anyone before marriage. But if she has or does -- we want to still be able to call her back home. If you know she is still a virgin, encourage her to see how beautiful it will be for her to share that with her husband some day. If she is not, encourage her to know that she can stop now and store up all the treasures of her physical affection from this day forward for her future husband.
I don't want you to think that you cannot share the negatives -- like STDs and fear of pregnancy and emotional suffering and all. But I want her to see purity as a positive thing -- not just avoiding the negatives! That's where the example of your life will come in. The joy she sees in you will communicate that message the most powerfully!
5) Have you read my book, Arms of Love? Have you suggested it to her?
I have heard back from many girls now, over this past year, who have said that after they read my book they were completely committed to living a life of chastity -- and to make Jesus their number one!!! It's not a hard read -- it's a novel. Most girls enjoy a love story. You can try that.
Be assured of my prayers for you and your friend!!
In His Most Holy Name,