Have you already tried calling the police on this guy? They may have him in their files and having calls documented with specific transgressions at least will alert them to the situation, even if they can't take direct action. It would be best, of course, if your friend would make the calls. I think the best you could do would be to document and report consistently. At some point, he'll slip up and cross a line, at which point the law will get involved. Moreso if he already has a criminal record of some sort (sounds like he might).