Several years ago, when I was still living in my hometown, I was working in a restaurant. I enjoyed the actual work, and most of my coworkers were pretty cool, but there was this one girl who did nothing but whine about how fat she was.
She was a high school student, a cheerleader who was tall and very thin. She had two friends working there, and they would rally around her and tell her how non-fat and beautiful she was.
Now, at the time, I was not as fat as I am now, but I wasn't thin either. I was in the process of becoming the size I am now, because I was willing to eat regular meals. Once I started doing that, my wrecked metabolism responded with glee, piling on stored energy like there was no tomorrow (or at least, no tomorrow that had food available). So, most people can imagine how aggravated the constant "OMG I AM SO FAT" was to me.
I finaly snapped one evening. I had heard it one time too many, and this time, she said it to me instead of to her ass-kissing friends. I said, "Look at you, and look at me. When you run around all day complaining about how fat you are, then what the hell are you saying about me? Do you even think about how it makes other people feel when you do that?"
She ran off, crying. Her friends came up and told me that I shouldn't have been so mean to her. I told them that I was tired of her insulting everyone else who was fatter than she was with her constant complaining, and that I wasn't sorry for telling her that.
After that, I noticed that she had stopped the behavior, which was a relief to other non-thin workers as well. I guess getting her to look at someone other than herself for a change gave her a much-needed reality check.
I want to add that, yes, maybe she was suffering from an eating disorder, or Body Dysmorphic Disorder. Maybe I could have been more sensitive to that. In my defense, I was much younger then, and didn't know about BDD; she sounded whiny and selfish to me, and having at least one person working there recovering from an ED, we just didn't want to hear it anymore.
I would not say the same things today--I'd probably be nicer, and take her aside and ask her if she's talked to her doctor about her self image. And maybe one could say that it wasn't any of my business to do so, but when someone is making their mental health issues everyone else's problem the way she was, it became our business.