I actually find it quite plausible. It's probably 550kcal or something like that, and I doubt she'll be eating a lot later in the day if it's during shows. I used to eat a breakfast like this when I was younger. Plus she's a dancer; she probably rips through calories really fast. It's just not that great health-wise.
That's a fair point. If she doesn't have time or the opportunity much in the way of lunch then it would make sense to 'load up' a bit on breakfast. But, yeah, I still can think of way better things to have for 550 cals in the morning, especially if she's maintaining a weight lower than her body would naturally have her be.