Here are my thoughts....
First, I would, indeed, require a lot more forthcoming information from your DD.
Second, unfortunately, you really set yourself up for this. As a rule, it is NOT good for parents to be that overly vested in their child's friend/boy-girl friend. It seems very obvious that you have done everything except 'stay out of it'. You are actually referring to this friend 'like your own child'.
Common sense dictates that this kind of drama happens with teen girls... and YOU took on that risk when you agreed to these plans so far in advance. Just because you personally took that risk.. and YOU became so overly vested in this other child... This should not obligate your own child, not one tiny bit.
Your child should have a right to end this relationship, NOW, if that is what she feels is best.
The whole thing is just SO convoluted, because you have chosen to make this friend, 'like your own daughter'.....
You may have seen this all as a very good thing... 'all right with the world'... but lines have been crossed. And, it is never a good thing for parents to be that overly vested in their child's relationships. It just isn't.
The fact that, even though your child has been insistant about this, it is still a personal topic of conversation between yourself and Jane, with you basicly taking Jane's side, without knowing the entire situaiton.... is a terrible disservice to your child.
The fact that you told your child, basicly, no matter what, she WILL spend her family vacation with this friend.
I am assuming that this whole thing is fully refundable at this point... but you still want to make your daughter 'obligated'.
If you want to wait until closer to cancellation window to see how things play out... by all means...
But, to obligate your young teen daughter to come in second and spend her vacation with her parents with another child in which she is embroiled in drama and ill will...
Not something that I, personally would ever, ever, consider.
Even if it were past cancellation.
I think you do need to ask yourself...
Who comes first here... your daughter, or the 'friend'.
I think that how you choose to handle this does send a major and undeniable message to your child.