Hi
@Maltose and welcome to the orums.
I may not be the best person to comment because I'm T1 and was diagnosed at 8, but different people handle their diabetes in different ways.
Also, some modern diabetic meds aren't suited to low carb diets.
If your cousin's meds are keeping his bgs down then his diet may not be too bad for him. Complications generally come after many many years of higher bgs. (Speaking from the experience of a T1 who has had more than 50 years of diabetic bgs.) Plenty of the late diagnosed T2s on here control their diabetes by changing their diets, but others use the medication route, and I think that that is a personal choice you have to respect.
Not sure what you mean by GFR ?
If you care about your cousin and want to help him then it may be more productive to ease up on the blame game. He may or may not be making the most optimal health choices, but his diabetes is not his fault. I would argue that it is caused by a combination of genetics and the modern food industry.
I buy my 96 year old dad (who has been T2 for 20 years) delicious keto cakes, which satisfy his sweet tooth (very common in his age group) but don't mess up his bgs.
Maybe you could go for walks with your cousin and bring delicious low carb treats with you?