That is all you can do. Some try antibiotics, and some who have a vet willing ro help can get hormone implants to stop laying for 4-6 months at a time. Leghorns being such egg laying machines are one of the many breeds that suffer from these things.
Some hens can develop fluid in the belly or ascites from heart failure or liver failure. It can be common in hens with egg yolk peritonitis. Ascites can cause a full tight lower belly, and labored breathing. If there is fluid causing this, some people or their vet use large needles to...
Does she have any fullness or tightness in her lower belly? If you have a helpful vet, you can get a few fresh droppings checked for worms and coccidosis. If it is is egg yolk peritonitis or internal laying, it may have come upon her gradually before any symptoms were noticed.
Welcome to BYC. I would bet that she might have a reproductive problem such as internal laying or egg yolk peritonitis. They can lose weight in the breast bone area, have problems walking preferring to sit, and diarrhea and a poor appetite can be common. Not a lot can be done other than to try...