Not sure I have any advice but a Facebook friend is doing a similar thing right now.
For months she was posting updates about her weight watchers group and how well she was doing, putting up photo's of her awards for hitting targets etc, sharing recipes and photo's of everything she was eating, photo's of herself in the mirror asking if we could tell that she had lost weight or if she looked like she had gained any.
Gradually it became all she ever posted about, she stopped talking about her family and it was all about food and weight loss.
A few people became concerned enough to speak to her when she started posting that she had reached her target weight as far as the group were concerned but wanted to lose a few more pounds just for herself.
The recipes were still being posted, and the photo's of food and if she ever deviated from her diet plan she would put up a status about how she had eaten a handful of crisps or a biscuit and now felt bloated and sick and was crying because she had over eaten on bad food.
She accused the people who spoke to her of being jealous too and she cut them off, saying her only real friends were the ones who she met through her diet group and understood her because they had shared the experience of going from fat to thin. Which was a shame as the reason I know her and lots of others on her friends list is because we met in a bereavement support group so we do share an understanding of loss with her.
Eventually she started posting that she was getting help for an eating disorder because she couldn't stop checking her weight or rationing her food etc and was terrified of putting on even a tiny bit of weight so was still dieting and doing quite an extreme exercise routine too. She was still losing weight and feeling ill because of it.
I think your friend sounds the same. It's great when someone can work so hard to change something they are unhappy with but sometimes it can go from being a healthy thing to an unhealthy one. But if that is the case with your friend, she won't want to hear it any more than my friend did.