This is the same fallacy that some people use about planes: "Traveling by plane doesn't result in more fossil fuels burnt because the plane would have flown anyway".
The way these things work is that after such an event the organizers assess whether there was enough food, and based on this they order food for the next event.
I guess you're right, it's a flawed argument. In fact, considering that I use exactly your argument to support/demand not eating meat, I guess it shows that I might just be going for a post hoc rationalization of actions that emotionally appeal to me in this case.
But just to play devil's advocate, might the charity decide that they want to take in a certain amount of money from an event and set ticket prices accordingly, thus passing the cost on to attendees? (The implication being that it's less ethically dubious to leech off of rich people than charities.)
Regarding the specific anecdote he gave: there was actually nowhere for him to sit down for dinner--so he never actually ate anything! (He did drink--or at least stand in line for such--though.) He would not have affected the "food consumed" post-hoc measurement.
The way these things work is that after such an event the organizers assess whether there was enough food, and based on this they order food for the next event.