Erm....she could have still been the main star. Sitting on a chair JUST to rap doesn't seem too hard. Eunjung had to go on stage to perform WITH a broken leg ffs.
If she really didn't want to do it then she should have told someone like the members or staff. If she really really didn't want to perform then leave during rehearsal. Not 15min before their performance.
Then again, don't even know if this article is true or not(most logical right now) so.....xxxx KKS.