I think speaking w/ the 17 year old won't do any good. She's already displayed lack of maturity and caution when it comes to such a powerful tool as the internet. I'd go to mom and let her deal w/ her daughter. What if you had been a stalker? Play that angle, oh and I'd only mention the security aspect of it. If mom wants to go check it out and see the other things, that's on her!
