What “investigate”?? Is being a jerk (either the racial epithet or the tip-stiffing) a crime now? Just what is it you think your “investigation” might uncover?
Apparently the guy was just picking up a takeout order. He wrote ‘none’ in the tip line, so a staffer couldn’t write in a tip for themselves and my guess is that act peeved the waitress who took revenge:
http://news.rapgenius.com/Devin-barnes-racist-red-lobster-receipt-denial-statement-lyrics