I faced this dilemma when my High school aged daughter and a friend wanted to go to a rock concert in Northbridge an a Saturday afternoon/evening. The other girl's parents weren't too keen on them going on their own and neither was I so in the end I went with them and agreed to stay discreetly in the background if they met up with any friends there.
The band, fortunately, while loud, was pretty damn good with a genuinely talented bunch of musos who knew their stuff and gave the crowd a first-class performance. Both girls were as high as kites afterwards and I earned a truckload of brownie points so it was all good.
No way would I have allowed them to travel in and back alone though, even in the early evening.
In Sue's case, I have to agree that, definitely, the other girl's parents should have asked you first and my answer would have been a firm NO!! The train, city streets, drunk/drugged meatheads and a skinny 13 year old in a mosh pit with said meatheads would send my protective Daddy instincts into overdrive.