Yeah dude you don't have to pay, it's not like she's doing you a favour by paying her way! Just say 'hey, I saw this festival, made me think of you and I'd love to go together - the tickets are £60 each, do you want to go?' + you could say if you really wanted 'I'd offer to pay for the both of us but I'm a little low on cash atm'
And them treat her to some little things like maybe drinks etc.