What a cheek. 
Funny how people tend to be more likely to pick somewhere expensive/something expensive when someone else is paying.
My DD used to do this. She had an 'allowance' from around the age of 13, to buy frivolities (we bought necessities.) Whenever I went out with her, I would sometimes get her to buy her own coffee and cake or something, just to teach her the value of money, and when she had to pay for herself, she had tap water and a 99p cookie. When I was paying, she would pick £10.00 worth of food and drink.
Same when she brought a friend with her. If I got them to pay for their own (when they were 14-16,) they'd spend £2.00 each, but if I offered to pay, they'd get fancy snazzy drinks with marshmallows, and squirty cream, (and always a large,) a pack of sandwiches, and a cake, and a cookie. Sometimes cost me an extra £20!
This is why I started to give her an allowance, and made her buy SOME stuff out of it. To teach her the value of money, and how much stuff cost!
It took a while! 
As people have said @beeleecc just tell her if she wants to go to somewhere expensive, you will go halves.