Judi James Chat 22/7/08
There's only one reason why she wants to go and stay at her BFs IMO. I may be wrong but don't know many 17/18 year olds who can control themselves either!
Your friend needs above all to talk to her like an adult (she's not but she'll think she is) and ask her straight out:- 'I'm totally jumping to conclusions here being as you want to stay at x's house, and tell me if I'm way off the mark, but please tell me that you're being careful? I love you and trust you but just give me something to go on to stop me worrying!'
Sometimes the element of surprise leaves them with nothing else to say but the truth.
I have all this to come with my kids but I do use this approach with my nieces/nephews and they do seem to respond to it.
Wish her good luck from me.