She didn't storm "off the stage," though. It wasn't a performance, it was an audition for the band that was to back her up. She "stormed off" after their lead singer took it upon herself (I guess) to "show her how it's done" which made her feel like she was being ridiculed. Which, if you read all the responses in this thread, was probably true, given that the OP is quite gleeful about how the singer showed her up.
Now maybe the girl wasn't all that good, or was inexperienced. I did my first professional live shows at 19, and I sure didn't know all the things I learned later at that point. There were no Instagram "stars" back then so I don't know what kind of expectations that brings, but as someone pointed out, it's unlikely she's had a lot of opportunity to learn how to do a live stage show.
I guess I just don't get why so many are so quick to say that she's the problem. If you don't want to back up an inexperienced singer, don't volunteer for the job. If you volunteer for the job, then do your job. Your job isn't to make her feel bad about her lack of experience; your job is to learn the material and play it and let the other chips fall where they may as far as she is concerned. Do this and you will get the paycheck. If you think you don't want to risk being seen onstage with an amateur singer, then maybe don't sign up for that.
We all started somewhere.
Perhaps I should go back and read it again. I gathered that OP's singer took over AFTER she stormed off?
Which is it @5andFretless ? DID your singer upstage the girl DURING the performance of the song?