Lindsey and I have been best friends for a decade. She's a lovely girl inside and out, and unfortunately has some self-esteem issues due to an emotionally abusive mother. 

Throughout our friendship, Lindsey's weight has consistently yo-yo'd. She would go on diet and exercise kicks for a few months, lose 15-20 pounds, and then quickly gain it all back. Every time she did, her awful mother would rag on her. Lindsey would come to me for reassurance that she's not the obese failure her mom has told her she is, which almost always included her asking, "You'd tell me if I reached that point, right?" I reassured her that she was fine the way she was and that she could count on me to tell her if she ever reached that point. 

Well, that point has been reached, and I don't know what to do. On the one hand, her weight has reached a point where I am starting to get concerned for her health. On the other hand, I know her mother is still harping on her for it, and I worry that having her best friend agree with her mom will crush Lindsey. 

TL;DR: Best friend of 10 years always said to tell her if her weight got out of control. She has reached that point, but I'm afraid to tell her because her emotionally abusive mother has destroyed her self-esteem. WIBTA if I was honest with her?