[Porsche-O-Phile]

Personally I'd say it ain't your place or your problem. Frankly I think you're pissing into the wind. Children of screwups are pretty much destined to be screwups. Your intervention, while well-intended, is virtually guaranteed to fail and likely to be unappreciated and resented by your niece and everyone else.

However, you're obviously a decent guy and despite the fact that you are almost certainly GOING to fail miserably in your efforts, here's the common sense reality - you cannot think for your niece. She has to WANT to fix her situation, just as it is with an alcoholic, drug addict or abuse victim. They have to want the help or they will reject it and go right back to the same situation. If she has approached you and convinced you she wants better than where she feels her life is going, then she can prove it by losing the phone, losing the "crowd" and hitting the books. I'd offer her a distraction-free household to get her life back on track - no friends coming around, no schithead boyfriends trying to get in her pants, no phone, no texting, just a good solid loving home, company, protection and whatever tutoring you can offer. If she wants it.

If the douche then comes around (onto your property) in that situation, far as I'm concerned it'd be a new ballgame and you'd be within your rights to wipe the floor with his ass ("home invasion" - hint, hint). If you were ever to chase him down on neutral ground or on his territory, you'd just make yourself look bad. If he comes around and trespasses on your property after being told "stay away" I'd let him have it ("self defense", protecting your home and your niece), then file for a R.O. just to add insult to injury.

Or you can just have a "mysterious, unindentifiable cloaked stranger" work him over a bit in the dead of night. While I do kinda' like the appeal of that and it'd no doubt be satisfying, it's just going to cause problems. Chances are he'd just use that as a sympathy play and would be too stupid to make the correlation between his being a douche and it resulting in his getting a beat-down - unless he was told at the time WHY he was getting the beatdown, and at that point you might as well call 9-1-1 on yourself for assault.