I was diagnosed at 14, but remember having symptoms beginning in 6th grade (I was around 12, I think). Anyway, I also was tall and stocky, weighing in at 140 lbs and 5'7 at 13. I always looked healthy, but my mom couldn't understand why I would be hungry, sit down to dinner and only be able to manage a couple bites before the pain kicked in, looking at the food would make me nauseaus, and I would lose my appetite. Gradually I went down to 115 lbs, and the doctors kept telling her there was nothing wrong with me, that it was all in my head, that I had anerexia, etc. Finally, the Dr put me in the hospital to run some tests (I think he just got tired of my mom b*tching) did an upper and lower GI, and that was when they discovered I had crohn's. They sent me to a larger teaching hospital to an actual GI, who did the colonoscopy.
Just keep pushing them for answers. I know this is scary, for both of you. Because of your history, you know in your gut that there is more to this. If she goes in to the hospital again, insist on seeing the GI on call. They will be more apt to listen to your history and run the appropriate tests. Good luck to both of you!