My DD is just back from a Disney trip with friends.
Hers was a bit different as it was an early graduation trip and her friend (the future graduate) was allowed to invite 3 girls along.
When the invitiation was extended they said DD would need to bring $$ for all her meals/snacks and souvies. Of course we're talking high school age here - so I think they did a lot of grabbing a quick bite here and there and not many sit down family type meals, so that made sense.
The friends' family was planning on covering everything else, although when we offered extra to help with travel and hotel expenses, they gladly accepted.
I really think every situation is different. Depends on your relationship with the friends' family, what you and they can afford, etc.
I do feel it's definitely best to have everything figured out and be clear about what type of financial contribution you expect up front, even before anything is said to the child, so the parents can make a decision about whether they are comfortable with the arrangements first. Too late for that in OP's case, unfortunately.
That said, if someone invited my child on a 16 day trip to WDW I would certainly NOT expect them to cover all expenses for that length of time.
I guess what I'm not clear on in this case is if the $600 suggested earlier was meant as money the friends' daughter was to take along and use at her discretion (ie: her extra spending money) or as a contribution to be given directly to the OP. I think you need to be pretty clear about that.
Maybe it's just me... but if someone said my child should bring along X amount for spending money - I'd think that was for her to use as she chose for extra snacks, souveniers, etc. and wouldn't automatically assume that park tickets would come out of that amount as well. (kind of like the earlier posters story about her friends' grandson)