Ik 2 people have told you to ignore it, I think this sounds like a really bad idea but tbh this whole situation is terrible. They are a dumbass for setting themselves up for disappointment on their birthday. I would first involve a mutual friend, an advice friend if you have one; just ask them how they’d advise handling the situation
Ik I’d hate to be left hanging, the anxiety from that would be as bad as the let down afterwards, but everybody is different and you will know your friend better than anyone online. This really is quite an uncomfortable situation to be left in so I wish you the best of luck in dealing with it 🤍