you may want to look into Clarityferm, which is a wine clarifier that coincidentally drops gluten counts to under 10ppm. Note it is not approved by the FDA for this purpose.
I have added it to several recipes that include gluten-heavy grains (barley, wheat, rye), and my sister and niece who have Celiacs, did not have any problems with those beers. I’ve had friends who have similar successes.
Depending on how strong their allergy is, they should probably just try a little bit and see if it bothers them first. My sister has a very strong allergy and it didn’t bother her.
Its great that it can work because sorghum beer sucks.
As an aside, many of the Celiac symptoms (nausea, stomach pains) are identical to reactions to glysophate (aka Roundup), which is sprayed on a lot of domestic wheat crops shortly before harvest. Also note that many Celiac positive people have no problem eating bread in other parts of the world…