I’m sorry to hear about your own and your Dads health. Has he had his liver and thyroid checked? A member of my family recently had a similar problem and began loosing weight, which it turned out was an over active thyroid along with a few other problems. It’s a good idea to check these things before starting the diet, especially if he will loose weight easily.
If he does go on the diet make sure he includes LOTS of oil to keep his calories up and stop him dropping under weight. Also check out different bread recipes with the allowed flours, snacks like buckwheat bread with (homemade) guacomole or aioli will probably be vital for him if he starts the diet.