I don't think you should say too much.
Let her know that her actions could have lasting consequences: sex equals possible pregnancy or STDs.
Other than that, I would just sit back and keep a close eye, without her realizing if possible, as nonchalant as possible.
When you see her making mistakes, try to guide her gently and give her the wisdom of your experience at that point. I think she will be more accepting of your counsel when you offer it and it makes sense to her and she can put it to immediate use.
Trying to give her a lesson on boundaries and acceptable behavior and unacceptable behavior pre-emptively before anything goes wrong in her relationship is going to be tough.
I have a 16-year-old son. He doesn't heed warnings well, my warnings and advice given ahead of time to head off problems seem to go in one ear and out the other, or worse yet, sometimes the warning doesn't even get in his ear, just goes right over his head.
But when he has a problem and I have advice that can help, he will listen. I was the same way as a teenager.
You know your own kid, if you think she'll listen and use the advice now, then go ahead.
[This message edited by wk55hn at 10:46 PM, July 23rd (Wednesday)]