I understand how you would feel a wee bit guilty, but I think it's a little misplaced.
If the situation were reversed, and YOU were the one that took it upon yourself to buy the tickets 'on a MAYBE', I bet you would not expect the other person to compensate you for your decision. She jumped the gun of her own choice. A heartfelt "thankyou for thinking of me, I'm sure you'll have no trouble finding someone else to go", will suffice.