You're right, I was a bit rash with what I said. And I have no problem with that she was let backstage, as you said, she helped him. However, I find what she said to be unneccesary and rude, especially if she did co-write his songs.
By the way, it doesn't seem to me that this was at Studio A, it looks like they went to a bar afterwards, which would make it seem more like a personal party.