I hardly see how he's hurting the charity by doing this.
It's not as if he's depriving the charities of money because he would have paid the large admission fees if he wasn't doing this; he just wouldn't go. And his presence there is not causing the charity to spend any more money by hiring more staff. There is the real cost of the food and drink he consumes, but depending on how the event is catered, if there is food left over, then he's not effecting anything, and if there isn't food left over, he's taking money out of the other attendees' pockets, not the charity's.
In fact, maybe he contributed to an enjoyable experience for the other attendees, which will make them more likely to support that charity in the future.
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.
All the drink at an event like this will certainly be on sale or return[1] so everything he drinks comes direct from the charity's pocket.
[1] i.e. the charity brings in X bottles of champagne and lets it flow freely all night. At the end they return all unopened bottles, and only pay for what was drunk.
It's not as if he's depriving the charities of money because he would have paid the large admission fees if he wasn't doing this; he just wouldn't go. And his presence there is not causing the charity to spend any more money by hiring more staff. There is the real cost of the food and drink he consumes, but depending on how the event is catered, if there is food left over, then he's not effecting anything, and if there isn't food left over, he's taking money out of the other attendees' pockets, not the charity's.
In fact, maybe he contributed to an enjoyable experience for the other attendees, which will make them more likely to support that charity in the future.
Utilitarianism FTW!