Albert Einstein once said "Insanity is doing the same thing over and over again and expecting different results". Basically, it's crazy to think you can do the same thing and expect to suddenly lose weight. Weight loss takes a change for it to happen. You just need to tell her, STOP complaining unless you are actually going to do something about it! You don't have to say it mean, just next time she complains ask "well have you tried a new diet or exercise? Anything?" and when she says no, tell her she can't really complain then.
You can exercise.
You can eat healthier.
You can eat less.
I used to be overweight and lost about 65lbs a couple years ago, and all I did was eat less. I didn't exercise more than normal, and I didn't change WHAT I ate, just HOW MUCH. My weight loss probably would've went faster if I did the other things, but I didn't want to

lol So just saying if your friend isn't gung-ho about doing ALL that change, she can just do one at first and it will help.
Basically, all I did was find out how many calories I eat per day (it was well over 2,000 and you shouldn't ever go over that!) then I slowly lessened how much I was eating over time. A good rule is not to eat unless you are actually hungry (don't snack just because you are bored or there is a free sample). Lessen her calorie intake and she should drop some weight, even if it's unhealthy food she is eating.