Prat of the problem is that "fat" is a relative term, AND it has a negative social connotation when used like this. Not having seen the girls in question, I don't have an opinion. As one of the above examples said, I wouldn't mind if the receipt said "old guy", but if it were a female serving, and the bill read "wimp", I might feel differently.
I do have to agreee, the guy is in a service industry, keeping the customers happy is the primary objective.