Our issue with this receipt isn't that someone's written 'cash' where the tip amount should go. As long as the diner really did give their server some cash, no harm was done. It's unnecessary to write it, but there's nothing wrong with it either.
We're a little more worried about the fact that based on the signature, the diner was Pac-Man. What did they order? Hundreds of tiny little pills as a starter, followed by a few ghosts as the main course?