Wilderness wildwater canoe trip – if he does not enjoy the challenge of working together as a family, you can simply drown him.
Weren’t summer camps invented for just this problem?
Summer camps come in all price ranges. From age 13 to 20, I often went to Dutch naturestudy camps from my Dutch Youth Nature Study Club. They were incredible cheap and basic. We usually camped near a farm in a nature reserve. The kids visiting these camps ranged in age from 12 to 25 and basically organized and entertained themselves around a common interest in botany, zoology, and just working outdoors. Personally, I have very fond memories of these camps.
There are even international volunteer camps where the kids can work on restoring old castles in France or something like that. Because the kids do some useful volunteer work, the prices are often reduced.
My experience is that when the camps have some sort of focus, like nature or culture, they attract more serious kids, and they feel good about the possibility to contribute or to organize themselves and tend to have a very constructive time.
If you just send a bunch of 15 year olds to a beach with nothing to do but drink beer, hold popularitycontests, and chase each other sexually, well, probably everything you fear will happen.
You’ve probably already tried, but I’d also ask for his input on the situations.
I can see how going to Disney World and/or spending time with the Fam at Club Med isn’t exactly going to thrill a 14-year-old.
If he still isn’t cooperating, leave him.
Ever thought of going to Vegas? That town is the epitome of cool for ALL ages.
I hope this doesn’t sound too harsh, but I think the “bring a friend” idea wasn’t the problem here. The problem here is that your niece is an indulged spoiled princess whose parents won’t stand up to her. IMHO.
(That is, I have zero problem with generous parents offering to bring a friend, but once it becomes a demand? Nuh-uh, no way.)
Seizing on the camping theme, have you checked out Fur Peace Ranch, run by Jorma Kaukonen of Jefferson Airplane fame? It’s basically a music camp, though not specifically intended for teenagers. Even though he probably couldn’t care less about Jorma Kaukonen or the Jefferson Airplane, there ought to be some good classes or workshops for him there, if he’s interested in lessons at this point.
Only negative is that it’s in the out-of-the-way hamlet of Pomery, Ohio, so getting there would be expensive.
I just want to add that you should be thrilled that he not only can play guitar, but wants to play it.
So he can’t be all bad 
Nah, you can’t be too harsh. She was spoiled and often a little bitch. I’m just stirring up the idea that if one teen acts a certain way, teen + friend might be even worse.
Leaving him at home is an interesting idea only if they can trust him. Maybe they can get a trustworthy neighbor or relative to “housesit” with the expectation that a curfew is expected for the teen during that time.