800 cal isn't enough for a 6'5" man?

[RockingMyFiftiesNot]

We are very close friends with another couple. The other DH is 6'5" so he can carry a lot of weight. He lost a couple of stone a few years ago following a heart scare, using a calorie controlled diet 1800 cals per day.

He has regained most of that over the last 18 months and is now following a shake/soup diet plan of 800 calories a day for the next 6 weeks or so. I said to his wife when she told me that 800 cals seems ridiculously low for someone his size to follow for 6 weeks, I can't see how he'd have the energy to do anything. She said she is very worried about it but he won't talk about it and is going ahead. I said I'd ask for opinions on here.
Thoughts?

It really depends on why he is on this extreme diet and whether he is medically supervised. I know 4 people who've lost over 50% of their body mass - my DB went from 26 to 12 stone, on a 500kcal per day regime. He was closely monitored. It 'cured 'his diabetes and he is like an different man. No longer addicted to carbohydrates, no longer drinking endless pop/beer. He glows with energy and health. It saved his life really. My dad and 2 friends have also had great success. For some people a total dietary reset is needed.

He should really speak to his doctor about it.

Many people do lose a tremendous amount of weight on Fast800, 5:2, 4:3 and that ilk but it has to be done with lots of leafy green veg, limited carbs (nothing highly processed) and plenty of fat and protein - meat, fish, tofu, cheese, butter, other full fat dairy and oils, plus eating your calories at least in a 12:12 pattern, but preferably 16:8. Some people much larger than me have done very well on it, and also people with multiple health issues, after checking with health professionals first. I went from BMI 28 to 24.9 in seven weeks, and had no hypertension or other health problems to start with but other people who have had high BP and type 2 diabetes have reversed it and have been able to come off medication they thought they'd be on for life.