Law enforcement?? Exactly what law is it that might have been broken here?
Do some googling. False accusation crime is one of many. Red Lobster is also open to civil defamation claims.
The negro waitress posted the receipt online, and also said she had been fired for posting it (which she should have been). The Red Lobster where she works said she had not been fired. That is what Corporate Red Lobster is investigating, whether she had been fired or not.