Don't worry about the extra ticket if she doesn't come along. All that matters is that the day itself is about fun. Hopefully she'll make it and we'd love to see her there; but it not, it'll be just fine.
If you've already told her about the tickets, please make sure she knows there's no pressure about showing up just for the money's sake. It should be about happiness and smiles or not done at all. What happens is completely in your hands.