If not at the festival, there will be another opportunity. So you want to empower her to make good choices whenever the risk presents. From what OP is saying, I would still buy the ticket, but with a chat first about why I had reservations. Which are about her safety, who are these people, will they mess up her life. So by buying the ticket I'd be trusting her not to do anything stupid/get in trouble. This and every such future event is going to be a test for her, and for me to trust her. "Let's see how it goes."
I don't know how to help her manage the risks
Keep her talking to you, so you've done well there already. At the right point in the conversation, when talking about the general situation, I slip in my opinions like these:
"Never drink so much that it makes you feel like doing stupid things.
Binge drinking is just plain being stupid, that isn't what fun looks like.
Just because your friends do something stupid, is not a reason for you to do it, too.
I will always hold you responsible for what YOU choose to do. And I don't want to worry you're going to do something stupid.
You don't know what you're being given or where that shit's been, so think very carefully about smoking or taking it.
Messing with that stuff could screw up your other plans in life."
They say that kids at lowest risk of substance abuse are the ones who have too many other plans; too much to lose.
(Sorry for stupid long post): the other one is E, it's still very popular. I think I'd have a frank discussion about how it makes people thirsty to their peril. Maybe that's a point to get her to read up about it so that if her "friends" ever take E, she knows how to look after them.