My brother was recently diagnosed with this disease.
If I’m not mistaken, the diet is like that of our hunter-gatherer ancestors.
I was diagnosed a few months ago. It is not a bad thing.
For me, it has been a Godsend.
I feel tons better. I’ve lost weight (some Celiacs are overweight, which makes diagnosis more difficult because doctors look for it in the underweight, typically.
I eat foods I would have likely never tried.
All of the symptoms I previously had are gone, after years of sometimes severe pain.
Good luck and good health to your brother. And consider the possibility that others in your family may have it. There is possibly an increased chance that they do.