First off, giving anything to a worthy charity is a plus, so I'm not one to criticize someone for giving less than 100%. As long as a model is upfront about how much goes to the charity, I'm fine with whatever amount she chooses to give. Even if she benefits from the event too, that's okay as long as she is clear about what's being given and isn't judgmental about what people tip for the event. Of course, she could just decide to make a donation and not make it a public event at all. Presumably the hope in making it public is that there will be more tips, allowing her to give more.
On to a couple side points.
The people who say that, from a member's point of view, giving directly to the charity is a more effective way of donating are absolutely right. All you spend that way goes directly to the charity (their operating expenses still apply, but you can't avoid that short of choosing a more efficient charity) as opposed to your token cost over 5 cents going to MFC. If you intend to spend time with that model anyway though, tipping her on a charity night still does the charity some good, and you get to support her doing something she believes in.
MFC's cut of how much of the model's earnings goes to charity is actually 0%. The model earns the same 5 cents per token as always. Donating 100% from a charity night will not cost the model more than 100% of the earnings unless, for some reason, the full amount can't be deducted from taxed income (also, I have no idea how non-U.S. taxes work). That said, I'm certainly not saying it's wrong not to give all 100%. This is still how the models make their livings, and giving away part of your paycheck is still generous.