I have a facebook "friend" who I went to high school who was obese until we graduated and is now a stick and constantly posts ridiculous messages about overweight people, sometimes TAKES PICTURES ON HER CELL PHONE of overweight people sitting at bars and makes nasty comments. This is in between posts about entering bikini contests and posting pictures of her showing her midriff and acting like she was never in her life fat (and let's not even get into a discussion on her plastic surgery).
She's a nasty person and I hide her on my feed, but every so often I go to her page and read her stuff just to get pissed off. I don't know why.
Point is, some people just have to knock others down to feel good about themselves. And maybe she's not competing with you, maybe she's just so excited that she's finally successful at a diet that she has to broadcast it to the world.
Just go about your business and worry about taking it off and keeping it off- something she probably isn't thinking about at all.