F.B.
Depends on the manner in which they are accustomed to traveling, and what expectations they/ you have regarding the funding. Have a conversation with him, I would probably plan on at least $75 to $100 a day to cover her costs.
If they are funding her trip, and you are giving her pocket money only then $25 a day should do it.
Best,
F. B.
PS- We were at Hershey park this weekend. A souvenier cup (which offers discount refills costing $.99 each) was available for only $9.00. Things can get expensive very fast.