It's a difficult process. I'm really not an expert, I did a psych rotation as part of my medical training, and I have some experience with dealing with PTSD thanks to UNIT, but I wouldn't claim to be experienced with this. I think the place to start would be helping her identify the underlying issues. The most commonly known cause of an eating disorder is poor body image, but that's only one of many aspects. For some it's about a need to control things, for some it's a form of self harm. It could be something else, it could be any number of combinations. It could be as a direct result of the Cage, or the consequence of years of problems.
The toughest thing will be the lack of support network. Because that tends to be vital. The people around her would need to walk that line, between being compassionate and yet firm with her.