My dad has diabetes, has trouble with a fewof his teeth having lost fillings and replacements keep coming out due to lack of enough tooth to bond to properly, and doesn't brush his teeth every day. Every time we go out i have a chewing gum and offer him one, but he doesn't always accept. Sometimes i have to outright tell him he needs to either have a gum or use the mouthwash he keeps in the car because his breath smells.
And it REALLY smells, like, makes you feel sick if he breathes in your direction or is talking to you and you have to face him. It baffles me how he knows he has bad breath, but doesn't brush daily or automatically have a piece of gum if he goes out and about in public.
Everyone knows morning breath stinks and stays all day if you don't brush etc, so how anyone can ever be oblivious to bad breath is beyond me. if it's gum disease your gums will surely be bleeding, or the dentist will tell you that you have it. Most health issues that aren't dental related will have other symptoms, so it really is inexcusable to claim ignorance.
I appreciate telling people is tricky, i find it so awkward with my dad both because he's my dad so i care about hurting his feelings, and i have anxiety. But they need telling, i'm sure they'd rather be told gently by someone earlier in the day, than find out later on and cringe at the thought of everyone they've spoken to all day.