I've been sitting here, wondering how I would handle this situation if one of my kids put me in your postition. This is what I came up with...
I would let the girl, and her family, deal with the airfare issue themselves. They get a credit, they can use it down the road. If not, that's their issue.
The park tickets are good forever. She can hold onto those or sell them to someone else..her choice.
The special party tickets I would pay her back for and then try to sell them to someone while in Disney, the night of the party. Those are not going to be usable on any other date, so you need to try to sell them yourself.
I'm curious why you think that the OP should be the one "out" the money for the special party tickets? If you feel that the girl and her family have the responsibility for dealing with their own airline and park ticket refunds, why not the special party tickets? Not trying to start anything, just curious what reasoning you use to say the girl is responsible for her own airline and park tickets, but not the special party tickets? Weren't they all paid in the same spirit.....that the girl was going, not really as a guest of the OP because she was paying her own way, but more as "sure come along".
If you or I had purchased special party tickets and then for whatever reason, be it our fault or someone else's fault or no-one in particular's fault, then we'd be out that special party money, and maybe other things as well. It's part of the risk you take when you purchase something so far in advance. How many posts do we see on DIS where someone had to cancel their trip last minute and is out all kinds of money (non-refundable airline tickets being the biggest one I see, but quite often the part tickets too). There is a post from the last day or two about a military family where they are schedule to leave in less than a week and DH's leave is being held up because of some paperwork, and because it's so close that family stands to lose a LOT in terms of airfare, deposits, etc. And it certainly isn't THEIR fault....DH put in for his leave quite some time ago, was conditionally approved (typical of military, they'll never say yes because the very meaning of being in the military is ready to go at a moment's notice......as far as the military is concerned he could be at Disney for a single day of his 10 day leave and be recalled and too bad for the family). But now, because some higher up is sitting on paperwork his leave is in jeopardy. Shouldn't someone somewhere be responsible for this family getting all their money back if they can't go now? The military certainly won't reimburse them....Disney may or may not, they certainly don't have to, but pixie dust tends to be strong over military families.
Perhaps the girl and her family in this thread could contact Disney, explain the situation and request a refund. Disney has reportedly given a few refunds, but it seems to be on a case by case basis, so maybe she'll get lucky, maybe she'll have to lose that $50 and chalk it up to the cost of a break up and making plans in advance. A relatively inexpensive life lesson really. And we all know if Disney says no, even though these say non-transferable a quick look at Ebay and you can see that's not stopping people from reselling them.
I still believe that the girl is responsible for her own cancelled trip expenses. OP shouldn't have to bear them as OP did nothing wrong. Neither did the girl really, but it was her money and her decision, and not she has to face the same consequences of cancelling a trip as ANY of us have to, and that includes loss of money.