Backing out on plans is not cool especially when it's for 'better things' (I consider work scheduling issues different or school commitments that came up different).
This sounds like my sister-in-law. Eventually people just stopped inviting her to things. That was her consequence. We went to Austin in 2015 and her and her mom got into a fight (which I had known is common on vacations when they've been on them together) and she finally let it slip she was really hurt that I was being invited and yet she wasn't (my husband and I were married at that point but we're talking vacations spanning mutiple years before that). It was explained to her that she would be wishy washy about being able to go and it was quite known that she would back out if better things came along. I think that was the moment she realized it (she was 23 at that point). I would have been invited regardless but she was finding out that she was actively not being invited because we could never count on her to actually commit.
Anywho back to the OP yes she should have stuck with her plans especially as those plans were made around her availability. Certainly she should be responsible on any costs incurred though such as flights, hotel costs, etc. I see no reason why you can't let her know you're disappointed because she's back out and not only that but she backed out because it was for something better in her mind. I wouldn't make it the hill to die on unless you really feel this is the tipping point in the relationship (like you need to have a heart to heart type conversation because there are other issues going on in the relationship). Otherwise I would just say "in the future we'll be making plans for X date and if you can come we'd love that but we're not going to leave it up to your availability anymore".
