In order to give her the money while avoiding to be seen as rude, you need to not assume what she wants.
If you assume that she wants you to pay her back and she, in fact, doesn't, you will put her in an uncomfortable position. You might hurt her feelings because she was happy to offer you those tickets but, now, you are treating it differently.
If you assume the other way around (she, in fact, doesn't want you to pay her back), the situation is still not great. You might put her in a difficult financial situation or she might resent you to have "trick her" into buying tickets for her.
To avoid both of those situations, I would suggest stating what you want and saying something like:
I feel bad for not buying those tickets and would like to pay you back. Is that okay with you?
First, you state how you feel. This way, when you tell her what you want to do (pay her back), she doesn't need to assume your reasons, she already knows. From experience, assuming someone reason and being wrong can lead to a lot of unnecessary conflict and resentment, so it's a good idea to try to avoid that.
Then, you check with her if she is okay with your solution (to solve the problem of you feeling bad). This way you show to her that it's not all about you and that her feelings also matters.
If she isn't okay with you paying her back, she can then say it and find another solution with you. If you didn't ask if she was okay with it, she might have not said anything because she didn't want to upset you but then resent you for not taking her feelings into account.
Note: I'm from France.