Shows how we all have different opinions on things but if a colleague of mine was organising a meal for 30+ people I would fully expect to pay for their meal although I accept the OP could of been clearer from the outset.
I would prepare 2 emails, first to said lady only very polite but saying you are disappointed in her reaction and no longer feel able to organise the meal and that you think she would be ideal to take over. In the second email to everybody say that you no longer feel able to organise the meal...