Oh dear. This is a case of a well-meaning server getting things very badly wrong. The human body comes in many different shapes and sizes. We should never assume that someone's pregnant unless we have evidence to confirm the suspicion.
At some point during this customer's meal, the server has clearly made a comment that implies she's pregnant - and she isn't. That's resulted in a sarcastic comment, and probably zero tips. We can understand why the customer would be more than a little upset about the comment.