I don't know if you should tell or not; will your parents abuse her physically or emotionally if you do? The thing is, 17 year olds cant be relied on to be responsible, and even less-so a 15 year old, and the risks involved in having unprotected sex are very high, both medically and emotionally. If I were her mother, I would want to know.
On the other hand, if you fear for her safety if your parents find out, the very least you should do is to take her to Planned Parenthood and ensure she is using birth control pills in addition to condoms (to reduce the risk of STDs).
Relax... I'll need some information first. Just the basic facts - can you show me where it hurts?