To be honest, I think you should sit her down and have a long chat with her about getting some psychological help. This girl is clearly trying to fill some sort of emotional void by having dangerous carefree sex without using any protection (as evidenced here:
http://turnyourselfin.blogspot.com/2...aking-it.html)
This girl is in serious need of an intervention. I say you do this: go along with the notion that you're going to have sex with her. Then when you guys are alone, sit her down and give her pamphlets that will lead her to help for her sexual addiction. It is evident that she has a problem and as a result, she is engaging in extremely risky behaviour. She might not want to read the pamphlets so I think you should contact her parents beforehand and show her parents this young woman's blog. I know it's severe but this is a severe situation.
On a related note, I think that by staging an intervention, you are no only helping her out, but your fellow human being. Her behaviour is likely to spread STIs to many others so it goes without saying that she's being extremely irresponsible.
Remember, there are lots of guys out there who are taking the easy lay, but I believe in you OP. You can rise above some simple biological urges and do what's right.