Both DD (18) & DS(20) will be at Reading, this is the third year they have camped though they have been there for single days before (we live about 8 miles from Reading)
The first year, when DD was 16 they were in a tent together, since then they have been in separate tents with groups of friends.
Like you I was worried the first year, although DD's boyfriend's brother was one of the security guards, so I knew there was someone who would be able to help if needed.
From what I've heard from both of them, it's pretty safe - of course, there will be people with drugs, but tbh, so there are any weekend in Reading town centre & probably most cities - if she's with a group of sensible friends, she's no more likely to get involved than in her home town.
I've not heard tales of any great violence, though there is a fair amount of petty thieving - DD's sleeping bag was stolen on the Sunday last year & she spent Sunday night wrapped in someone's coat.
It does help of course that we live so close - the first year DD came home on the trian every morning for a shower! but she hasn't bothered with that since.
If you have an old mobile phone it might be better for her to take that rather than one that might be a target for thieves - DD always takes my old one, thoough if she uses it much the charge is unlikely to last the weekend.
As for food, the festival is within a walk of the town centre, where food will be cheaper - last year DD seemed to exist on pasta & Marmite and cucumbers!
If you're worried about her finding her tent, get her one of these that she can paint in an easily identifiable way (though if she brings it back, I wouldn't read all the comments it will probably acquire if you are at all prudish....)
Otherwise, I'd recommend one of these as they are easy to carry & put up - packing them away does need quite a bit of practice first.
Oh, and tell her not to pitch the tent near the toilets - DS did this the first year because he thought it would be convenient - they were woken up all night by people falling over the guy ropes & the smell was pretty unpleasant after the first day!
I'm sure she'll love it & be fine.
If there's anything else you need to know, just ask (I'm normally RustyBear btw, I'm only The LadyofShalott atm becacause the Curse has come upon me, in the shape of MIL staying...)