I reckon that she doesn�t have a contract yet. She has discussed the possibility of performing, and has stated that in principle she is interested. The contract would be along the lines of �I�ll sing at your event and you will pay me �x� but she hasn�t agreed to the fee yet, that is still up for negotiation. It might be slightly different if she always works for a standard fee, but from the sound of your post that is not the case.
Verbal contracts are legally binding, but the agent would have a hard time proving anything in court, particularly if his standard contract allows a cancellation period. Remember it is for him to prove breach of contract (if one even existed) not for your daughter to disprove it. She could always sign the contract (checking that the cancellation clause is in there first) and then cancel according to the terms.
I reckon she has 3 choices.
1. Suffer it and do the show
2. Nogotiate hard and ask for the same money she would get for the other show before formally agreeing
3. Call his bluff and cancel.
I guess it depends on how much she needs this agent for future work and whether he will bear a grudge.