I do want to add that after reading your update, my original post probably sounded crass or very hard.
Yes, like you, I probably would have doted on Jane some of the time or most of the time, since the parents clearly were not.
The few things that I would have done is, yes, John is the owner of the property. As for the bon fire and child being in the house along a good distance from parents, since he would be the one held liable, he should have spoken up to Jane's parents and stated that someone needed to be with Jane or in an area closest to Jane where if the child arose, she would have a parent close by in a strange, not child proofed home. (And I would not child proof my home for the future. That is not my responsibility. I might do some things, but I would not go all out in my home.)
I don't mind Jane so much in the adult activities, as long as she was being tended by "her" parents. Sorry, but they decided to bring their child along, I am here to enjoy my outing, they can take care of otheir own child at this time.
And, I would have not let Jane's parents dictate how I will spend my vacation. Sorry, I don't plan to do child activities. My child is home. I might have done one activity or more, if I so wanted to do so. But, for them to demand that everyone participate, I don't think so.
It seems to me that Jane's parents did not want the responsibility of their own child. I understand that sometimes as a mother myself. Hey, we all need a break. But, I am sorry, on my vacation, I am not your babysitter for the whole week. I don't mind some time here and there, because that is the way I am, but not all the time.