LOL dammit i wish i could lean out dancing...WAY more fun than tredmilling!!
Sounds like she has a fast metabolism and/or her activity level (energy out) is in a major surplus compared to her food consumption (energy in).
If you can (or she can), track her diet. Food journals are cheap and as simple/basic as they are, it is an incredibly powerful tool for learning.
I imagine (not to stereotype) that many dancers have issues with food because of obvious reasons. Tough situation.
My suggestion is to spend a week or at least a couple days tracking her diet, then bumping up the calories slowly until you reach a sort of "equilibrium" for energy in - energy out. I do not suggest suddenly adding 1000 calories a day, even if that's what she needs. Not only will her body likely go "WTF?!" and store it (if she is overtraining and undereating), it will do a number on her mentally.
She also needs to learn about eating to fuel althetic performance and health (sorry maybe she is already---but i'm lacking some info from your post so i don't know what she is currently doing). If she has a bit of an eating issue, the reasons are a lot deeper than just acheiving an aesthetic image, and would take a longer time to resolve/work through. So be patient and supportive
Maybe she doesn't have any sort of eating and it's a simply just figuring out the math, so to speak. Either way, i still reccommend doing a gradual caloric increase (of the healthy variety!!) ;-P