I agree with the suggestion mentioned in the last couple of posts. This girl got your daughter understandably flustered and cancelling the party was a handy response she came up while in that state of emotions. Clearly, it was "overkill" given that cancelling the party is punitive to herself and the other four guests.
In the future, your daughter should understand that, if someone is asking for something that makes her uncomfortable and she doesn't know quite how to express her feelings about it, there is nothing wrong with saying that she needs some time to think and she'll get back to that person. Then, she can write out her thoughts or role-play with the nearest handy, sympathetic person until she feels comfortable and confident in the answer/response she is going to give.