Awkward situation, I once had to discuss this with my own mum, it wasn't easy. She's also diabetic by the way.
I suppose the main question to ask is how do you think she's managing with her personal hygiene? Maybe she's struggling physically, especially if she's feeling tired, or maybe she's lacking motivation if she's feeling down etc. You've known her for a long time, think about how her personal hygiene is now compared to previous years. Once you've excluded this, it's time to consider other causes...
Has she got any infections? I'm thinking about things like UTI's, intimate areas, skin and mouth/teeth. All of these have quite distinctive smells. I've smelt all of them over the years (used to be a nurse), so I got quite adept at sussing out likely problems from smells! Have a look out for things like if she's going to the loo more often (possibly a UTI), is she covering up more (she could be hiding a skin infection)? etc. What's her blood glucose like? Could it be high and causing her to have bad breath associated with DKA? (usually smells more fruity/chemical-ish smell in my experience).
I know through experience that this isn't an easy conversation, best of luck with it x