They're in their own home, the rellies, and were going to have dd's friend alone, then decided to invite dd too. Their own children are grown up, so I guess dd is useful in that she's company for friend.
The invitation mentioned paying her own flight and spending money, now I'm panicking should I have offered a contribution to the housekeeping?
this happened to us recently - I gave a contribution equivalent to what we would have spent on the child during a week away bearing in mind eating out, ice creams, entrance costs to places etc etc. I think you should at least offer to contribute - they may say no!
But she's taking her own spending money so I'm assuming the rellies aren't planning to pay for everything for her when they're out and about? How old is DD?
I think the suggestion that she write and thank them is an excellent one - old fashioned good manners go a long way IME. You could call her and suggest she buy some flowers (if she's old enough) the day before she leaves for her host as well?