<shudder>
Respectfully, aren't you a mandated reporter of child abuse? I don't mean this in a snarky way but if the parents won't call the police, maybe you could use your mandated reporter status (if you are one) to report to CPS under the rationale that what was described was certainly abusive, even though the abuser is not a family member.
Thinking it over, I don't think her parents are completely wrong. If it were me, I'd get her a second cellphone number but retain the old one so that the stalker doesn't realise she has a new number. And then leave the old phone with LE so they can just take the messages directly.
The parents may think this is just a very twisted prank but even if that were so, I think it would do the prankster a world of good to have a little face to face time with LE and the court system.