It'll pass, but it will be tough to start with. It's like a kind of grieving process. I was in utter denial at the start, thinking it was all wrong and I'd be eating all my faviourite stuff very soon. Then I went through terrible anger, mainly at the doctors who didn't pick it up and my GP who kept telling me I was just depressed. I put in a complaint and for a long time wanted to persue compensation etc. I was angry at my parents for ages because they would hold birthday gatherings and completely forget I couldn't have cake, or because mum would say "want a biscuit?". I got angry at my friends for barely understanding what gluten was. I was furious at food manufacturers for putting wheat in stuff that should not need wheat in it, such as sausages and in chocolate and on chips. I was generally quite angry, for some time. Then I cried a lot, mainly in supermarkets. More than once I broke down sobbing going past the bread aisle and when family went to McDonald's.
It probably took a few months to get past all that and accept it. Whilst it still feels unfair, I get that life is unfair sometimes. Most of the time I'm so used to gf substitutes now I don't even notice. We don't have gluten in the house which makes it easier. I've found lots of new favourite foods I never would have known about. And when we have family celebrations I get a Tesco free from caterpillar cake (which is yummy) and even better, I pretty much have it to myself! I've made peace with the whole thing, the failure to diagnose, the lack of understanding, the fact that it's forever. I've pretty much forgotten what gluten stuff even tastes like and it isn't such a big deal any more.
At least now you have it confirmed your DD can start feeling healthier and stop doing further damage. Maybe she can find other young ones in her position - I know there is a children's page with coeliac uk. Focus on the positives - you can discover new foods, there are lots of great substitutes, and things are getting better and better for coeliacs. There has got so much more stuff in the supermarkets even in the two years I've been diagnosed. In the future things should just get better and better. But as well as the positives, allow her to grieve, as she may well need to. 