Aw dear I hope she gets well soon. she's too young for this
I was/am diagnosed with Gastritis in December and I am on my way to healing. It's all about diet really. Don't worry eventually she will get rid of it.
If you found her stool being too dark or any sign of blood take a stool sample to H. Pylori test. Don't worry it's nothing serious just a bacteria that could be the reason for her Gastritis, and if the test is positive then he will add another medication to Zantac.
I know it might sound gimmick but if your daughter likes pomegranate make her eat a lot! I have it daily no matter what I eat I feel better after that, it prevents that "nausea" and "fullness" feeling that she feels.
Don't make her eat any hot food, leave it all till it's warm.
She can enjoy jello and pudding if she likes them, they sooth the stomach inflammation.
I am posting a long detailed story of my gastritis and I hope it helps.
God bless her