I was once in an exercise class where at every session a young, very thin, beautiful woman would rush into the class right before it began, stand next to one of the mirrors and pose this way and that and exclaim, "Oh, God, I am soooo fat!!!" Then there would be a long awkward silence in which all the other women, mostly overweight women would try to ignore her. One day, I found myself screaming, "Arrgh!!! I CAN'T STAND IT!!! DON'T DO THAT!!!!"
She turned away from the mirror and stared at me. I was embarrassed; it had just popped out of me.
Suddenly, the whole class burst into spontaneous applause. Later, several women thanked me in the locker room.
So...maybe stop talking about it and next time she does it, let out a loud scream of rage and say "STOP THAT!!!"
If doing that a couple of times doesn't work, you may have to live with it. A friend of mine who has weighed around 300 lbs for many years, as did her husband, used to have to deal with her mother-in-law who would rush into their livingroom and twirl around smiling with her hands on her waist singing out, "SIZE TEN! SIZE TEN!"
