Depending on my understanding of why she let me down and didnt want to go, even though she had agreed previously, she might by now be an exfriend!! It is definitely down to her to deal with the ticket not you, and it would only be a saving grace if she admitted that she had let you down and gave the ticket to whoever could use it. Dont put yourself in any bother doing her dirty work for her. If and it would be a big IF I felt that this had not been at a whim but couldnt be avoided I might in the future go with her to something else, but more likely, would remember her cavalier attitude and either let the so called friendship fade, or just buy your own tickets to things, then if she choses to buy a ticket and also go the the show, you might meet her for a drink in the bar at half time, but certainly find your own way there, do as you want to do, and never rely on such a person in the future!!