I don't have experience with celiac specifically, but I have been dealing with IBS stuff over the past year. When I initially realized something was wrong I had been enjoying bud regularly and didn't have a lot of pain. But as I stopped smoking I started developing more intestinal pains. So as I focused on my diet I gave up most gluten... the only glutenous grain I ate was barley. I started eating lots of fibrous and green veggies (fennel, zuchinni, spinach). I got most of my proteins from lentils and mung beans but would occasionally eat almonds or cashews in my meals. I also started taking a lot of probiotics and herbs used in Ayurvedic, Chinese, and Western traditions. This experience has actually encouraged me to do research on herbalism and I would highly recommend looking into this area if your friend is looking for alternative approaches to healing.
In retrospect, though, I think the most beneficial aspects of my diet have been (1) staying away from nightshade vegetables (tomatoes, potatoes, eggplants, and all peppers including spices except for black pepper) - the reason I don't eat these is because they contain solanine which is an alkaloid that can cause really bad inflammation of tissues and mucus membranes - (2) cooking with the right oils, because rancid or spoiled oils are terrible for your intestines, and most widely used oils are not very heat stable - I really only use ghee or clarified butter as it is the most heat stable oil but a non-vegetarian has more options with oils derived from animal fats - and (3) staying away from sugar, aside from a little honey, as sugar can really irritate mucus membranes.
As for medicinal marijuana, I've found it be incredbly useful in reducing or eliminating pain and promoting healthy digestion. The effects are best felt when ingested, but stil help if smoked.
I wish your friend the best of luck in finding better health!
