I think the "friend" acted atrociously, but from the OP it seems very unclear that the friend understood that the tickets had already been purchased and paid for by the OP. I get the sense the friend thought the plans were not set in stone, and that should they decide to go, they would show up and buy tickets at the venue, not grasping that this would be a foolish plan and not grasping that the OP thought that the plans WERE set in stone and had already bought tickets. Again, the "friend" was very rude, but in the future the OP should, as many posters suggested, be far more clear about who was purchasing tickets and when and get money up front so all people whom she expects to pay actually have "skin in the game" as they say.
From the "friends" perspective I bet she was thinking, "Sure, we discussed going to the concert but I decided not to go. I don't understand why she wants me to pay for a ticket to something I didn't go to!" I bet she did not realize the OP had already purchased tickets. Not that this affects whether the "friend" was rude or not - she was either way - but just as a heads up to the OP so she does not get stuck again.