I like to keep the hens with their flocks, unless they are being pecked or bullied. If that is happening, I keep them protected inside a dog crate with bedding, food and water. Even if one is sickly or dying, they are usually more comfortable with the other chickens.
You can pull down on her wattles to open the beak, pop a human calcium tablet into her beak, and she will swallow it. Has she been laying eggs? How old is she? She might be laying internally or have ascites or both from her upright posture. Cancer is also possible. Salpingitis causes them to lay...