If she 16 and sensible she'll be fine.
Gigs are one of the safest things to do as a single woman imho. Admittedly festivals are slightly different, but as long as she's got her head screwed on she'll be fine.
I've camped in a tent by myself at numerous festivals before with groups of friends without problem. And I had a bit of a stupid wild year at Reading, where I turned up on my own with no ticket, only £15 to spend after I'd got a ticket and no tent, and still survived. And £5 of that went on the worst bottle of Gin I've ever drunk in my life. I was a bit of an idiot that year, but people helped me out and looked after me.
Yes there are horror stories (and I could probably tell a few), but I have to say, the majority are down to too much booze and a lack of common sense rather than really malicious stuff.
One trick I always used to do with my friends, was look around at who we were camped near, and with caution be friendly with the people around us. Look for groups of couples and try not to camp next to a large male only groups. Don't be too trusting, but be friendly as it can help protect you as the people camping next to you generally are as worried about their stuff as you are. It means that the people around you know whose tent it is and to a certain extent will look out for you. It stopped us getting our tent turned over at Glastonbury one year...
Festivals are for the most part friendly places and people are there to have fun, rather than cause trouble or commit crime. If you look at the size and number of people at festivals, the crime rate for the population and age of the population is actually very low - though you'd be forgiven for thinking very differently due to newspaper headlines.