Hello, and pls know that this post will be of limited use (I'm not diabetic).
1. From the Healthline piece I read, 300 is high. Can you ask his onc., "Why do you think this glucose level is OK rather than a problem? What info am I lacking to understand this? And, if I've misunderstood -- if you *do* consider 300 to be too high / to be something my friend needs to deal with, then I'm asking you to please impress upon him the need to test his blood and take insulin. He's not complying, and my efforts to get him to comply are falling flat."
2. Who are the "others" you refer to (those who consider 300 to be problematic)? Other healthcare providers? Can you explain to them that your friend is ignoring their expertise and that you're asking them to try another approach to get through to him?
3. I don't say this lightly but: If your friend is going to rely upon you for so much, then he MUST agree that *your* stress level matters -- in part altruistically (presumably he cares about you) and in part selfishly (you can't take care of him while you're depressed / stressed / angry / hopeless about his self-destructiveness).
. . . . . . . I understand that you don't want to present an ultimatum to someone who matters so much to you -- I really do get that -- but the bottom line is that if he's going to self-destruct, then you have to decide whether you're going to set a boundary and keep it: "I love you, but I cannot continue to take care of you while you refuse to take care of yourself. If you'll agree to participate, then I'll help you get the help you need, including psychotherapy -- but I cannot do this alone, and I *won't* do this alone. You have to decide whether you're going to participate in your healthcare, which means doing what the doctors tell you to do -- and refusing to deal with your diabetes is going to make your life even harder, which by extension will make MY life harder, because I'm the one you rely on."