And which lesson is that?
I can't tell from what you've said, but your communication might not have been clear. She might have thought that you were checking with her family before spending your own money, not before committing them to spend theirs.
Before you decide to be offended, just remind her. "Tomorrow is the last day to return the tickets with no penalty. If you're not going to be able to pay me for yours, then I'll need to return them. Please let me know by [XX] time." If you leave a message, confirm her receipt of your message, and then leave it alone. If you don't hear from her by the designated time, then just go ahead and return the tickets. You don't even need to mention it any further.
Don't be upset about it because that is a waste of feeling. And it only serves to garner ill will between you. Stop basing your expectation in assumption, and just use CLEAR COMMUNICATION. I didn't read anything in your post that told me that she should have been clear about your intentions. Don't hold miscommunication against her. Do you truly believe that she would have so blatantly--and, quite literally, in your face--offered to buy these same tickets for someone else if she'd known that she was delinquent in her payment to you for this exact same thing? You've seen her five times, and it's never come up? Come on....