What a rude girl, I can't imagine "demanding " more tip money! That is tacky! I agree , a tip is not a "mandatory fee" so the girl was out of line!
Don't go crying in " well she needs that money" tough, I need money too. and I have on occasion gotten a smaller tip than expected, but if the client hands it to me with a smile then I understand that they were happy with my service but are either underfunded or cheap. I don't EVER demand more.