Is 24.3 mmol/L (437.4 mg/dL) really normal?

My father recently developed Type 1 diabetes as a side effect of pancreatic cancer. He won't let anyone speak to his doctors so we are getting all this information through him and only him. He insists that his readings are "normal for him", that his doctors are happy with it, and that he can eat whatever he wants (mainly chocolate and pizza and ice cream) in any amount he wants, as long as he takes his insulin. I am very concerned, especially as he seems to be losing his eyesight, but he won't hear any arguments, and keeps saying it's not all and we need to respect his privacy. He lives alone and I am concerned about him going into a coma.

So I'm asking if this indeed can be normal, and if I really do just need to stop worrying and let him be.

Thanks in advance for your advice.

EDIT:

Thank you so much to everyone for responding, I feel much better now. As I've said to a few people, the only person I've managed to speak to is one nurse who told me I had to be incredibly proactive and stay on his case or we would find him dead on the floor, hence my concern. I was so torn between wanting to respect his wishes and trying to stay on top of it like the nurse said. I really feel like a weight has lifted from all the responses telling me that the effects of the high will not really be an issue considering his prognosis. I'm now going to focus on helping him enjoy what time he has left ❤️

Thank you all so much