Who do you think is in the right in this situation?

on the 9th november, i booked 3 of my friends tickets to a concert which cost �54.75, and they said they would pay me back. one paid me back promptly (the next day), one paid me �50 the next day and then �4.75 on 13th december (after being asked on 3 occasions to give me it), and one 'friend' hasn't paid me anything at all. i've texted her and asked her for the money and she doesn't answer the texts or phone calls. it's getting on my nerves, and my parents are starting to get really annoyed as well, and it's MY money she owes! they're trying to get me to sell her ticket but that would cause a fight so i'm really confused. bearing in mind that she has had over a month to pay it, PLEASE give your opinions; who's in the right? should i sell her ticket? the mum of the friend who paid the next day said it was disgraceful that she hasn't paid me, seeing as they were expensive and the amount of time shes had to pay, and i'm inclined to agree. i'm going to london christmas shopping this weekend, and because of her i have about �10 to buy presents which is nowhere near enough (i dont have a job because im 13). please answer this! thanks for reading, and answering x