I don't see why she wouldn't fit in with Glasto... Isn't Glasto basically just a pop festival now with a few lesser-known acts from varying genres?
I mean, looking at some of the previous line ups in recent history, that's all it seems to be to me. Sure, there's some actual bands that play guitar, but they're the usual pop-rock suspects
If you want a festival with 'guitar bands' Download is what you want since rock is where guitars are featured very heavily.