Help,where's best to sell T in the park tickets?

My DD has been left in the lurch by her so called best friend who has not paid for her T in the park Thursday to Sunday camping ticket and is no longer going because she has found a boyfriend. My DD cannot find any other friends to go with her due to work commitments and is devastated as this was her 18th birthday present.
Now it's down to damage control and how much can we salvage from this. I have no experience of reselling festival tickets, either pricing wise or where is the best place to list them. Any advice would be very much appreciated.