@Mugnika type 1 diabetes isn't caused by being fat, but by a genetic problem.
Keto will help manage his blood sugar. Low card diets were originally invented for diabetics.
Ignore the vegans.
Reducing Carb intake can help people with diabetes
Reducing fat intake can help obese people with diabetes
Exercise has a GLUT 4 effect which helps people with diabetes
Exercise uses both a higher absolute amount and percent of carb for metabolism the higher the intensity of the exercise
Reducing Carb intake hinders Anaerobic Glycolysis making long duration high intensity exercise more difficult
Ask health professional about insulin dosing based on meals & exercise
@Mugnika Dont let your friend try the Keto stuff will make your friend worse keto destroy your health even in short run a whole Food Low Fat Plant Base Diet is the best bet to restore the health your friend lot of grains, fruits, vegetables and starches will help your friend to improve his health