Parenting Teenagers (12-18) Support Group
This community is focused on the joys, challenges and concerns faced by parents of teenagers (12 to 18 year olds). The major areas of child development include: physical development, perception and sensory development, communication and language development, cognitive development, emotional development and social development.

1) you have to allow and trust her parents to parent her.
2) talk to your son (or better yet have DAD if he's around) about having sex with someone he doesn't care for. Having some self respect and that includes respect for others. I don't necessarily know if he cared or didn't care for his ex gf. But remember sex is natural and not always bad. If you have a daughter put it in perspective about that happening to his sister (he may never have sex again).
Good Luck!
If that is your question, I think you should deal with your son as you see fit, and not tell her parents. That will just cause a world of problems. Hopefully, your son and this girl were smart enough to use some kind of birth control. How to deal with your son? Well, at 17, I am guessing he is going to do what he wants to do. I would reiterate the importance of respecting women, and practicing safe sex, but beyond that, it's really out of your control. Good luck.