High quality feed, scrambled or boiled eggs, leafy greens, lots of fresh water. Give her a wormer! If your other birds are a normal weight and one isnt, she could be egg bound, or full of worms that are robbing her of her food. Worms are contagious, so be on the look out with your other birds for signs of worms.
I would seperate the hen from the others and give her lots of really good food and lots of healthy treats, like cooked eggs, cauliflower, seedless grapes, cantaloupe, oyster shell, cucumber, romaine lettuce, etc. Make sure she is getting the right amount of protein and vitamins.