In the specific case mentioned here, where state and Federal law enforcement won't take charge, why shouldn't she find a professional investigator to confirm who the perpetrator is, then use the same tactics he has used against her against him?
After all, law enforcement refuse to take action right? So it appears that law enforcement wouldn't care if she ensured that he is kept out of the picture.
(for the record, I'm playing devil's advocate)
It's not preferable but if it's the only way to get results... Nobody deserves this kind of treatment.
There are already services like this, they just don't have an LLC or an App :-)
Also, if someone is dangerous enough to intimidate ... well, you don't want to intimidate them and risk having them escalate back. Following that logic, you only end up in one place.