I dunno, Ozzy, this kid is what...16 years old? Not exactly an established master sexual predator. By all means notify the parents and scare the kid, get the word out, but he hasn't broken any laws here. There's a pretty long shot between saying or thinking these things and doing them. At least with things as serious as this.
Jeztor, I know what you're saying about not breaking the law, and that's why I have not called the police, but Ozzy raises a good point. People tend to ignore these things for too long and slaps on the wrist tend to give confidence to molestors to do more. As an FYI, at age 16 one has had plenty of time to become quite predatory. These things often begin with the predator being molested in their youth, becoming violent as this kid did before puberty, and sexually violent later. The victim becomes the victimizer. No excuse but it is not at all a long shot between thinking and saying these things and actually doing them. People like this will often test the waters with a new victim by saying provocative things to see how they will react. If they are quiet as my daughter was, they often become victims.
Just to clarify things, my girl did not spend the night. This happened during the day in just four hours. I dread to think what might have happened to her if she slept over and he had access to her. In talking about this with other parents, we have heard more than one story of girls refusing to go back but not talking to their parents about what happened. I'm glad that my daughter had the guts to tell me about this and I will be far more careful about where I let her go.