I have a relative who has a list of food ilnesses including egg, nut, wheat, soy and gluten allergies. As far as I know, he cannot eat anything that has the bracha "hamotzi". How can he fulfill the mitzvah of eating challah during the Shabbat meal? (FYI - He does say the full "Birkat Hamazon", relying on the fact that he is "satisfied".)
He can use oat bread which is gluten-free and is biologically unrelated to wheat or barley.
Not all poskim consider oats to be hamotzi (that's its own question) so you should mention to your relative to ask his rabbi. As DoubleAA pointed out, he would probably prefer to use oat matza instead of attempting to eat oat-bread.
There is no obligation to eat any food on Shabbat which harms you or is painful, even if that means fasting completely (OC 288:2). I obviously don't know your friend's medical details, but if there is nothing he is allowed to eat then he is simply exempt.
Rav Moshe Sternbuch in his Teshuvos V'Hanhagos 7:23 writes that bread without gluten is a mezonos,and a bracha mein shalosh is said after eating it,even if one is koveah a seuda on it. However,this is only talking about a healthy person. He says a Choleh who cannot eat gluten should wash,make hamotzi and bentch on such bread since for such a person its considered pas. CYLOR for lmaaseh.
Conclusion: eat gluten free challah.
You can drink enough wine or grape juice to constitute a seudah. I do this regularly at a kiddush if there is nothing there without sugar in it (pretty common, actually).
Here is a decent article specifically about your question, which also supports my answer.