I'd ask him for the reason.
Or are you having cans of sweetened green lemon ice tea? It may be the GP was thinking you meant this, not simply green tea with lemon.
I don’t see why it would be bad for a type 2 diabetic, it’s low carb and low calorie, unless you’re adding loads of sugar of course. I just googled “is green tea bad for diabetics” and there was nothing to suggest you should stop drinking it. I would ask your GP why he said this.
Its very effective at treating genital warts apparently. However green tea contains significant amounts of aluminium and they recommend no more than 5 cups in a day, and less if you have kidney disease.
I suppose green tea can raise blood pressure in some people, so it could be something like that, but in any event, your GP is an idiot for not telling you why. He may just know nothing about diet, and be completely wrong about the tea. A lot of doctors seem to think you should fight T2 with a food pyramid of statins, baked beans, bananas and oatmeal. Maybe he's one of those...