As you're discovering, one of life's most frustrating experiences is to try to help someone who is headed toward almost certain disaster - and then have them ignore you. Jesus had the same experience; He once cried out, "How foolish you are, and how slow of heart to believe all that the prophets have spoken!" (Luke 24:25).
Why is this? Psychologists point to various reasons why some people seem determined to act unwisely and put themselves in harm's way, even when it brings them great hurt. Some people, for instance, are rebelling against their upbringing, or have a hidden conviction that they aren't worthy of true happiness and deserve to be punished.
Others are so desperate for love that they'll do anything that promises happiness - even if that promise is false.
But the Bible says there is a deeper spiritual reason why we act this way (and we all do, to some extent). That reason is sin, which is like a terrible virus that infects our souls and cripples us morally and spiritually. This is why we need Christ, for only He can take away our sins and help us live the way we should.
Pray for your friend, and keep urging her to face the consequences of her decisions. But most of all, urge her to turn to Christ and open her heart to Him. God loves her, and once she discovers this truth her life will never be the same.