She broke your verbal contract to go on holiday together. Therefore, she is in breach of conract. Contracts do not be have to be in writing, they can be verbal.
Tell your so called mate that if she wants to waste valuble time and money on a small claim, she will be sorry.
If you refuse to refund, and she does issue you with a summons, then you have a right to dispute this. In the mean time, make a documented statement of all the things that happened and when they happened.
If you are still worried, go and see your local CAB for some basic contract advice.
Jo