One thing I hope you will do (if you haven't already) is tell her that you love her–and because you love her, you hate to see her head down a road that she will almost certainly regret later on.
Admittedly, people who are tempted to take this route seldom stop to think about what they're doing. Everyone else is doing it (they say), so why shouldn't I? But almost every week I get at least one letter from someone who ignored the advice of their parents or friends and decided to live with someone outside of marriage–only to have it end in heartache and betrayal.
You see, one of the problems with this kind of relationship is that it lacks commitment–and without commitment there can be no lasting security. It can end at any moment–and when it does, it almost always leaves pain and anger in its wake. But God has a better way–the way of purity and commitment. He wants what is best for us, and He knows it will not be found on a path of casual relationships or self-centered pleasure. The Bible is clear: "It is God's will ... that you should avoid sexual immorality" (1 Thessalonians 4:3).
Pray for your daughter–not only that she won't make a wrong decision about this matter, but also that she will turn to Christ and make Him the center of her life. He alone gives us true joy, and He alone can give us the courage to do what is right.