I am not diabetic but my Mom is and I have a hard time to get my mom to watch what she eats.
I feel for you. I'm lucky enough not to have it, but my mother does. I try to cook for her as much as possible, in a sort-of "forced" healthy eating (for us both, I can always use more moderation in my own diet), but during holidays you can't tell your mom that they've had enough cookies and deserts xP
Ok that's mostly a joke, she's pretty good about moderating herself nowadays.
(Also, even though she's never told me she was happy I worried and managed her meals for her, she DID say as much to my sister, who told me. So that's something - I know she really appreciates it, despite the occasional complaints.)
Dunno if meal service is an option for you, but if you can, people always appreciate you cooking for them. (Can always start slow too, by just decreasing meal size slightly or swapping out "bad" stuff for better etc. Even a few good meals and snacks a week is helpful!) If you can't cook for her, maybe help her plan out a meal plan and weekly grocery shopping? I know my mom was a bit against it and dismissive when the doctor and dietitian tried, but I picked up the pamphlets and chatted about it and she started coming around. Sometimes it just takes a different person saying the stuff at a different time.
Also, get pantry organizing things. Boxes and plastic containers. Don't pack them full, put just a few servings in each. Hide the worse stuff behind the better stuff (not all in one place) - out of sight out of mind. It sorta works, if you don't get so annoyed looking for a particular item you throw everything on the floor
My dad is diabetic and I don't believe he has made any changes at all.
Maybe you should casually chat with him about it? A kid's allowed to inquire about their parent's health. Maybe he's already managing it fine with medications, or maybe he needs a nudge from knowing you care and are willing to help to make a few changes.
My niece's dad was just in hospital due to complications. He refuses to make any changes though (I don't really know him to talk with him about it, especially with my sister having divorced him, but it obviously worries the girls a lot).