Perhaps she has never had to think about weight control up until now. You mentioned she used to excercise alot, but did not mention if her eating habits have always been the same. Is it just the exercise that has changed or have her eating habits changed too.
IT is easier to influence a person by setting an axample rather than criticising. When you start undermining you loose your authority. This is difficult I know, because I have a son who over-eats, and it is hard to watch someone you love possilby dammage their health.
I get the impression that she must enjoy sports, so why not be tactful and ask her to join you in some form of exercise you can both enjoy. Exercise, as well as burning up energy will improve her mood and she will be less likely to turn to food. That could be part of the problem, perhaps she is missing all the endorphins and stimulation she use to get from exercise.
She may already want to lose the weight, but sees it as a very difficult task. Once she starts working out again it could turn her attitude around. It will also force her to aknowledge what she has been doing to herself by eating junk food, since exercise puts a person in touch, and heightens their awarness of themselves in a physical sense.
Another method could be to try going on a juice fast. I have noticed with my son that the moer junk food he eats the more he wants it;it fuels the addiction. You could make out that you are interested in this for your own health, but ask her to join you. YOu know her best, so you can work out the best approach. Sometimes once a person begins to loose weight it greatly motivates them to carry on.