Well, you can't actually physically prevent her.
I don't think I'd try - it just makes it more attractive-sounding. I might talk up the general rankness & naffness, the pressure to take crap drugs, the likelihood of pervy older blokes, the local cops jumping all over under age boozing, the likelihood of spending all week in a sweaty tent in the rain bickering with her mates...
Actually, I did Reading Festival in '86 & all of the above applied! We had a great time
Seriously, I'd sit her down, tell her that it's ultimately her decision but that whilst you trust her you'd be worried sick about her getting into dodgy situations through no real fault of her own, you think it'll be a bit rubbish anyway, & you will in no shape or form be funding it.
If you know her mates' parents, tell them all this too!
Then shamelessly bribe her, if it's affordable? Book a family holiday somewhere in hailing distance of halfway decent clubs, let her bring a mate, & agree that there will within reason be a relaxed vibe re: boozing & curfew...?