Hey CSOG, I'm sorry to hear your daughter is having such a hard time. These gut issues are a beast for anyone, let alone a teenager. It sounds like your daughter is in a pretty similar boat as I've been over the past year. I had several ER visits as the result of severe dehydration from sometimes 15-30 episodes of diarrhea per day, or extreme pain - very similar to how she described them, like I was being knifed or something equally awful - that also sometimes woke me up in the middle of the night. And all they could do was the same fluids, narcotics, etc. that your daughter has inevitably been receiving.
Things did start getting better, eventually. Not immediately and not noticeably at first, but over the course of the past two months I started noticing less episodes of pain and fewer days of bad diarrhea. My doc said it could be a remission of sorts, although they are not sure what is causing it, so it's unclear if it's just going to keep getting better or if I'll slip back into the nastiness. I'm holding out hope for the former.
Is your daughter under the care of a good GI doc? Have they talked about any other medications besides narcotics? It's good she's on a low-residue diet. Calcium carbonate, Lomotil, or cholestyramine might help the diarrhea. As for the pain...has she tried regular doses of anti-spasmodics like hyoscyamine or dicyclomine? These are not nearly as strong as Vicodin or Percocet but if she takes them regularly (i.e. four times a day) she may notice fewer days of great pain.
I don't really have any advice, but I do sympathize, and although it's not likely she'll just wake up one day and be symptom-free, there is a chance she'll gradually start improving on her own. Is she still able to hang out with her friends, keep up with her hobbies? Keeping her life as normalized as possible is the key, even though it's obviously hard when she's ill.