She knows she's fat. I'd say nothing, unless she pushes the point and seems to want a response. Then it's probably safe to just mirror what she says, starting with a comment like, "I hear you saying that (…)" or "You've expressed (…) often lately." She may not be aware that she's been heard.
I do have to object to a previous poster's comment that "she knows she's eating junk." Isn't that just automatically shaming those of us who struggle with more weight than is fashionable? I used to believe that until I had personal experience that proved to me that it's not necessarily junk food causing the problem.
I've never been a junk-food junkie, and consume something sugary or fatty POSSIBLY once a month, usually less. The first time (post menopause) that I gained a serious amount of weight was when I went vegetarian for 2.5 years. It was a crazy-healthy diet with lots of whole grains (thus plenty of carbs), just wrong for my metabolism; I was always tired, cold, hungry, and weak. This came on gradually enough that I failed to recognize it was diet-induced, and I spent 2 of those miserable years actually under-eating and over-exercising in an attempt to stop gaining. It only made me colder and fatter.
When I reluctantly started eating meat again (just 3-4 oz. daily) and cut out most of the grains, I felt better and started losing weight, even eating more total calories. I've lost 2/3 of that weight now (45 lbs), but it's taken 4 years.
I know many middle-aged or older women in my boat. Many of us have gained considerable weight while eating responsibly. Most of those in my social circle grew up long before most junky or processed convenience foods were widely available, and are good, healthy cooks who understand nutrition. Some genotypes are simply more susceptible to weight gain in general, and often in specific areas.
Please, ladies, check your assumptions about fat people. Sometimes, perhaps often in the case of us older women, those assumptions can be wrong.