Strictly no glass - decant spirits etc into plastic bottles. From memory you can't take drinks into the arena (including sealed containers, water bottles) - you will be searched and these confiscated.
For this reason it's better to pre-game in the campsite (be sensible!) as it will reduce the amount you need to spend in the arena 
I understand the rule is no bags bigger than A4 in the arena - bumbags a good shout. Agree don't padlock tent, it marks it out as a target. Valuables at bottom of sleeping bag sensible if not investing in a locker. Try not to leave valuables unattended/ don't take anything you're not prepared to lose. Unfortunately campsite thefts are a fact of life. Security and onsite police very hot on it but chances are belongings won't be recovered.
Is she arriving by car or public transport? If parking or dropping off in Green car park (Kings Meadow) there is a big Tesco there and a free boat shuttle (!) to the festival Green Gate (very central in the festival).
There is also a ginormous Tesco at Portman Road which turns into festival central - they transform all the displays into camping equipment, alcohol and crisps, it's really something to behold. Promo stands and stalls outside and refreshments, it's an entertaining spectacle
. I'm sure the locals avoid it like hell!
This article mentions free shuttle buses to Sainsbury's etc:
shuttle buses
Richfield Avenue and surrounding roads are filled with vendors - Domino's etc, the works. As a pp said it's a good shout to get in with all your bags, get wristbanded, pitch tent and venture out for supplies on Thursday daytime.
Hope she has a great time, I love it 