Ignore it. It may frustrate you, but it has nothing to do with you - she's a grown woman and will decide what to put in her mouth.
I completely feel your pain, though. For a while my grandmother was living with me and she's a severe diabetic and has high blood pressure. After five years of failing eyesight, sugar levels that are unheard of, passing out in stores, hospital visits, etc, she STILL eats all day. You know what she eats? Animal crackers (sugar) and fruit (sugar).
I'd make a beautiful veggie-rich homemade chili, for example, and she'd tell me she couldn't eat it because of her ailments. Then, she'd eat a bagel and an entire can of tomato soup, adding water to dilute it because of the sodium, but still eating the entire can!
It's one of those things. You just can't worry about what other people are doing. You can express your concerns, offer your knowledge or even cooking assistance, but sometimes it's time to disconnect emotionally. You can't control anyone else but yourself.
