Unless this boy is completely half-soaked, he will no doubt wish to know what the book is about, since he is going to school dressed up as a character from it. And how do you answer? - fob him off with a bit of nonsense? Why put him and yourself as parent in that position in the first place? What is the point?
How does this advance his education? How does it help with world book day?
The school really had very little choice but to exclude hm from the day, as the children would no doubt have been asked to talk about the book they represented. There is no way the school could have involved him in doing that or there would have been parents up in arms - and rightly so. I do not feel sad that he was "left out of the day" - I have no doubt that he would have been OK about the chance to bunk off for the day.
What a very silly attention-seeking mother. It would be nice to think that she had learned her lesson, but she seems to be lapping up the publicity. I would not want her for a Mum, nor would I wish my child to be in her class.