I know I would rather hear, for example, that I should stop eating so many sweets rather than that I have incurable diabetes. (Just a random example).
A person might prefer
to hear that. However, if the person actually has
diabetes, it's not actually helpful to have someone with no medical training or familiarity with one's symptoms or tests to tell
the person that.
I grasp that some people would rather be told they have the diabetes and that it is out of their control, but not everyone feels that way.
I'm not sure why being given a diagnosis indicates that something is out of their control. Personally, I would rather know what is wrong with me than be given inaccurate reassurances that I'm fine (or worse, implied condemnation that I am being a hypochondriac in getting medical advice in the first place).
Even if the OP's friend were acting from the best of intentions, for someone with no knowledge of what is going on in the OP's health to tell her to discount the information given her by a doctor is wrongheaded and potentially dangerous (even deadly). "Just stop eating sweets, you'll be fine," is horrible advice if, say, someone really needs to take insulin.