I dont know the specifics but will ask him, he isnt massively overweight, probably has about 30 to 40 pounds to lose, that why I was questioning it, I feel at the age of 52 he will lose significant muscle mass which at his age isn't ideal
So I reversed my T2D by losing ~10kg.
I wasn't "overweight" by the BMI measure to start with - BMI of 25 point something. It's visceral fat around the liver and pancreas which matter, and a better rough guide to that than BMI is the waist-to-height ratio. NICE guidance says height should be at least 2X waist, probably a pretty good target.
If you want more background on the losing-weight/losing-visceral fat approach, Prof Roy Taylor of Newcastle University was lead on the clinical studies and he has a bunch of information for both lay-people and scientists at
https://www.ncl.ac.uk/magres/research/diabetes/reversal/
Although the studies and the NHS protocol are couched in terms of rapid weight loss via a very calorie-restricted short term intervention using soups & shakes, it's not really an essential part of the approach. Roy Taylor has said & essentially all experts I know of involved with this say that a more moderate approach can work perfectly well. It did for me, cutting back eating somewhat and grinding off maybe a half a kilo a week for a few months. So that's something to consider.
Along the same lines, a research program partly funded by DUK called NewDAWN is looking at other approaches besides for soups&shakes for long term sustainable weight loss.
https://www.diabetes.org.uk/about_us/news/funding-new-type-2-diabetes-remission-research-NewDAWN Not much immediate help, but does reinforce the point that the important thing for this approach is to lose weight; doesn't matter much how you do it or how quickly you do it, to a large extent.
You have a valid concern about losing muscle mass. I'm not sure how well the NHS soups&shakes program deals with this, but it's important. Basically, weight loss should be combined with resistance training to minimise the impact. It's one reason why it's best to pursue weight loss with the advice & under the supervision of a practitioner with appropriate skills.
Wish I'd known that at the time! I lost a bunch of lean mass, which has taken a long time to restore.