A mix of money and friendship is always tricky. A while ago I lent money to a friend who promised to pay it back as soon as she could. I didn't expect it immediately, but I did get annoyed when she went on holiday before paying it back, as well as buying new clothes and replacement items for her house (things like a new microwave because the other one didn't match her kitchen).
IMO, if money is owed it should be repaid before any non-essential spending. Clearly my friend had a different opinion, so the lesson I learnt from that was that if I lent money in future I would specify when and how it should be repaid. Maybe your friend feels that as you can afford to go on holiday you can afford to pay her back?
I think that the concert situation muddies the waters, as you were only going because your friend wanted you to. It would have been kind of her to at least offer to pay half, or you could have said you could only go on those terms.
Finally, I think it is never appropriate to decide what others can afford. Your friend may have a good job and be careful with her money, but that is no reason why she should not be paid back as quickly as she would have been had she had none.