@eaturveggies how old is she? Tbh all the advice above about sticking with your mates, looking after each other, preagreed meeting points if separated, old Nokia and a power pack, knowing who to approach for help if the worst happens are the absolute best advice.
Don't be afraid to approach Oxfam stewards in their hi-vis, they are there to help. If you get an unhelpful one (rare) try someone else, they are all over the arena - every stage and stage perimeter, outside toilet blocks, all disabled viewing platforms, arena emergency exits, NME signing tent. The most knowledgeable will be the supervisors denoted by orange and yellow hi-vis (with supervisor in black letters across the back!) who also hold radios.
As it gets busier over the course of the evening the stewards at the edge of the arena (toilet blocks, emergency exits, outside every music stage/venue) and those at the arena info point are easier to find in the crowds and generally under less pressure if you need straightforward advice like directions/line up info etc. The ones working inside the stages and manning disabled platforms will be preoccupied with crowds and medicals especially during the popular acts and headliners.
Welfare is a very welcoming place to go if you've overdone it for any reason or need a little quiet time to regroup. The staff are wonderful.
Campsite stewards are present in every campsite and have an obvious hub 
I don't really bother cooking at festivals these days - usual fare is cereal bars/breakfast biscuits/porridge pots/fruit, crisps/sandwich/pot noodle/hummus/ disposable BBQ/ meal purchased from a trader at night 
Grab fresh supplies from the supermarket on Thursday/Saturday morning. As staff we have access to boiling water but this info mentions The Salvation Army in Green campsite (not far from arena) provides boiling water for a small donation if she doesn't want to boil her own!