Under 16s need someone over 21 to accompany them to T in the Park - who has to have filled in guardianship forms. The rules are here.
Ds3, who,is 17, went this year, and that is the youngest I woild behave considered letting him go - it is a huge number of people, on a relatively small site, with all the issues that that brings - there is a lot if drinking, and drugs too - ds3 saw people who had obviously overdosed, being carted off by ambulance. Population-wise, it is Scotland's 5th largest town during the Festival, and my sense is at there are fewer problems and less crime per head than there would be in an actual town, but the confined site makes it more of a worry - or would if I were sending a 15 year old.
Plus it is pricy - £200 for the 4-day camping ticket, £30-ish for the Refresh ticket, travel - ds3 and 40-odd of his school friends hired a bus - I think that was £35-£40, and all the other things you need - we spent about £75 outfitting him and he spent more in top - and he didn't need to buy a tent or sleeping bag - we had the latter already, and he and a friend shared his friend's tent. A horrifying amount got left behind - the tent and the sleeping bags, clothes (though he did take old/worn-out/outgrown stuff), camping chairs, and lots of small stuff like drinking bottles, toiletries etc.
Maybe if you do get to talk to her about it, you could go through the costs and ask her how she will afford it (assuming she has found a 21-year-old guardian) - that might put her off.