I'm going to leave for a music show without my friend
Why would you want to go to a music show without your friend?
I want to spend time with my  other friend.
It's wrong to ditch your friends if you know they wanted to spend time with you. You should talk to them and see if they would like to join you.
Why would I tell her that? I have a ticket and I'm going.
Because you told her she could come right? It's wrong to change plans midway through.