Very well said. I would add that in my experience 10+ years on and off Keto that those that have a blood type O (40-45% of the population) seem to have it very easy and can get into ketosis very quickly. That has been my experience. However, I have a good friend who is an MD and recommends Keto for some of his patients, but he has struggled immensely to get into ketosis and was consistently having issues. He believes it is a protocol short-term (6-12 months) to address metabolic issues and that not all people respond to it.
All that’s required to get into ketosis is to cut way back on the carb intake. Perhaps his problem is with becoming fat-adapted? I could see that being a problem for someone.
Unless he’s simply having trouble giving up sugar, grains, and starches. That can happen, too.