ETA: You have changed your post to include more details. My answer to your question is still the same, but I think you have some other issues to deal with. You have one issue with your husband and another with this friend. First you and your husband need to get on the same page as far as your finances. Even if you can afford to give away $6000, is that how you want to give it? What are your MAIN priorities? Are your children's college accounts and your retirement fully funded? If not, wouldn't this money be better spent there? If they are fully funded, what about charities that are near and dear to your heart? If you have the money to spare, isn't there some organization that could really benefit from a $6000 donation? You could impact so many lives!
As far as the friend, from your added information, I sense resentment on your part. You don't want to give this $6000 trip to her, in part, perhaps, because she doesn't seem to be as giving to you as you may be to her. She made a comment about not missing a free class in order to pick up your kids at a time when you really needed her. You felt she wasn't there for you, so why should you "give" her a $6000 vacation. It seems that you feel the relationship is very lopsided, with her giving nothing, or else giving very grudgingly, and with you potentially giving very much. Do I have that right?
You and your husband need to get on the same page about how and when you give money to others. Then, you need to spend some time thinking about friendships and what you value in those relationships.
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ORIGINAL:
Did she have to buy a plane ticket to get to your destination? If so, and you get a refund on your trip, she would be stuck with that airline ticket, unless it is a refundable one. So, if she's already purchased the ticket, and it's non-refundable, then I think you would compensate her for the ticket or the change fee she'll incur if she makes a change to use the ticket at another time.
Outside of that, no. You don't "owe" her anything. It was a nice gesture of you to ask her to accompany you, but things out of your control have changed, and this trip is just not going to happen as a result. A good friend will and should understand that.
I would NEVER expect anyone to give me a vacation, but that's me.