This is an old thread, but didn't the accused racist get a lawyer and is planning to sue? Do guilty people normally do that in these cases? Just sayin...
I believe it constitutes slander and he has grounds for a civil case if he wants to pursue it.
He can bring in a handwriting expert to testify that it's faked and they'll go step by step over everything to prove it. It's so poorly faked that I can tell it's a forgery just from analyzing the picture. If they can get the actual receipt and analyze it under microscope that'll be further proof. And they'll have old handwriting examples of his to put up against it that will be even more proof (for example, there are no other "g" there except in the slur, his will most likely look completely different).
They'll also be able to take old handwriting from the waitress and if she did forge it they'll be able to prove that.