Would you say that a 1200-calorie diet for a ~240-pound man who plans to be physically active qualifies as a crash diet?
It's getting in the crashy zone for sure but he's never going to lose fat at a rate he's happy with on a diet over 2,000 calories per day. I mean, 2,400 is his daily maintenance amount so he's got to cut into that pretty hard to lose real fat.
Unless he is a contestant in a fat loss show or has a medical reason for the speedy rate of weight loss i see this as completely irresponsible even under a dr. supervision. Since you seem to suggest that this is a good idea, would you care to break down his optimal macros for us? Fat consumption to maintain healthy hormonal levels alone would eat up nearly all of his allowance.
Your challenge will be to get enough protein to maintain muscle mass and stay at 1200 calories. Speaking from experience, you'll certainly lose weight, but at, IMO, to high a cost to your overall heath. It's a positive that you're having some medical supervision, but you might want to consider giving yourself more time to reach your goals. Just be careful, especially if you're over 50.
As I'm sure you know, crash diets don't work. Chances are, he'll lose some muscle, in which case it will be easier to regain the fat. Next, if the caloric intake is low enough, it will negatively affect his thyroid function, reduce his testosterone levels and increase his cortisol output, all of which will impede his progress and possibly reverse it in due course. Anything other than a sustainable lifestyle change is almost certainly transitory.
So just avoid the question of whether to sacrifice fat or protein? I would have thought that with your extensive knowledge on the subject you'd add something more insightful than just eat a lot less cals to lose a lot of weight quickly.
The bottom line is that he's trying. With 58lbs of fat to lose, he needs a framework to follow and Atkins will give him that. It may not last forever, but it's worth a shot instead of doing nothing IMO.
I'm surprised you had a doctor tell you to eat at that much of a calorie deficit. I dropped from 225 lbs to 195 a few years ago on the keto diet. But I was eating 2000 calories per day and losing 1.5 to 2 pounds per week. I was pretty active at the time. I think my average maintenance would've been around 2800 to 3000 calories per day. Average calorie intake for maintenance varies depending on gender, age, activity level etc. I would check an online calculator and make sure you aren't eating at more than a 1000 calorie per day deficit. That can be really unhealthy long term. Good luck to you. Enjoy the bacon.
I applaud his effort and I do wish him the best. But it shouldn't be an all-or-nothing proposition of trying to undo years of neglect in a matter of weeks or a few months.