M.D.
I think you need to ask the auntie for the itinerary and straight up ask what he'll be expected to pay for so there is no confusion. If my nieces were visiting me, I'd plan to pick up just about everything, but also expect they would have some spending money for souvenir extras ($100 would be plenty for two weeks).
But, if she's expecting him to actually pay for his dinners when dining out, pay for admission to an amusement park, or if he's scheduling his own stuff like going skating and not with her family, then he'd need a lot more $.