Wow! You pushed a hot button! Unfortunately, diet is highly individual. What bothers one person does not bother another. Plus the docs will tell you that diet does not cause a flare ( a lot of IBD sufferers would disagree with that). However, diet during a flare will most certainly make your daughter more comfortable. Typically a low residue diet is advised...all the stuff we are told generally not to eat...white refined flour..low fiber foods. No seeds, nuts, popcorn.
This all said, I am a huge fan of diet but what works for my daughter (vegetarian/vegan) will not work for many others. Some people have good luck with FODMAP or SCD. The anti inflammation diet is quite the craze these days and may be worth checking out. For others even if they do not have celiac low or no gluten works. It is a process of finding what works...like everything else with this darn disease.