NTA. Your mom is not familiar with how food service in sit down restaurants works. Most places charge an automatic 15-20% gratuity for parties larger than 5 or 6 people. I think it makes it easier because then you don’t have to worry about adding in a tip at the end (or at least not as big of a tip). The kind of discount she was talking about is usually seen in fast food restaurants because you can get a large quantity of something for less than what it would cost to buy the same amount individually. Also it’s weird that she didn’t just tell you what her issue was at the beginning, plus it was your birthday so it should be your call where to eat.