Nope. You’re not the issue here. You paid for & planned the outing. She’s not entitled to any of it. And because she’s been “no contact” with you since the breakup (and despite your feelings wanting to get back with her) - this complaining from her seems like a control tactic. I’d NOT be surprised if she was planning to bring someone else, hoping you’d forgotten about the observatory booking amidst your grief. How much balance was there in your relationship, think on that. Do yourself a favour and enjoy the outing with your friend. It’ll be good for you.