the graduate is right, for two reasons.
First of all, if tennis "stars" really wanted to do something, they would have anonymously donated their AO prize money, I'm sure three of them alone would collect more than you and I and 5,000 others could collect.
Secondly, is our society so rotten that people would only help if they get something in return, in this case a silly tennis exhibition?
That's your opinion.
Some stars might decide to donate some or all of their prize money, privately or vocally - that's their prerogative. It doesn't in any way make this pointless - the more that can be done to help, the better.
Secondly I don't think society is so rotten that people will only help if they get something in return. The millions of dollars of donations that have been raised around the world are a testament to that. But there's no harm at all in doing something like this - it draws attention to the issue, might encourage those who have not donated yet to do so or those who ordinarily might not to do so by coming along to this event.
I just don't see how any of this can be a bad thing but I do have a way underestimating peoples' capacity for cynicism.