I am still friends with my high-school computer science teacher, who is now probably 75 or so. In the last 18 months he has lost 70 lbs, from 240 to 170. How?

I am still friends with my high-school computer science teacher, who is now probably 75 or so. In the last 18 months he has lost 70 lbs, from 240 to 170. How? He is diabetic and got a continuous glucose monitor. Before every meal, he'd check his sugar. If it was 150 or higher, he skipped that meal. If it was lower than 150, he'd eat, but half his usual portion and he tried to limit carbs. No exercise program. He lost 70 lbs in 18 months, has extremely cut his insulin use (with his doctor's blessing), and his goal is to get off all his diabetes medicine, again with his doctor's blessing. She said he will need to keep insulin on him in case his sugar spikes over 280, which can happen in diabetics no matter what they are eating, so she says. Just posting this as a possible aid to people who do not believe they can lose weight.