I am IBS-C and have had many hospital bouts due to constipation/impaction before I finally (hopefully) figured things out. Yesterday was one year since my last hospital stay (for three years I was in about every three to six months includng three surgeries).
How much insoluable fiber does she eat? I had to basically delete insoluable fiber from my diet. It just made the constipation worse (pain, gas, etc)no matter how much water I drank or exercise I got. All those fiber supplements and raw vegetables and fruit (even eating soluable fiber first)just "cemented" everything in my gut. The last diagnoses (after many) is slow transit colon which certainly makes the most sense. I take Zelnorm and Milk of Magnesia daily.
I eat soluable fiber including cooked fruit and "smashed" vegeetables daily now. I don't eat any dairy-all soy although too much of that can give me lots of gas. I have learned to live with all the funky stomach noises (especially since I am not in pain anymore)but then I am 49 years old. I can only imagine that it would be very tough as a preteen.
I snack all day long, very few "meals". I drink lots of water/peppermint tea/fennel tea/other herbal teas and cranberry juice.
One of the best tricks I learned when in pain (mine was always-and on occassion still is- located on my left side, sometimes near the center below my belly button and/or under left ribcage)is to lie on my left side with my knees drawn up to my chest. And take long, slow, deep breaths. Maybe you can help her with the breathing, it does help to have a coach. It also gets the mind focused on something other than the pain.
Good luck. Hope you find something in all this information that helps.