Two questions:
Was this originally ordered as “to go” or did they get seated at a table and - for whatever reason - change their minds and decide to take the food to go?
Could there have been a 3rd party that wrote the offensive word on the receipt after the patrons had left and before the waitress picked up the receipt?
It’s a hoax. She got ticked that she got stiffed and had a co-worker write it.