I assume you are referring to third spacing of fluids. Sometimes, doctors wait and let the body work things out on their own. A common treatment is albumin. It is given intravenously and it is a protein. It helps to pull fluids back into the blood vessels and helps to raise blood pressure. Often when patients do too much third spacing, their blood pressure can become lower. Albumin will help correct this.
Yes, you can give Lasix to treat third spacing if kidneys are unable to remove extra fluid from the body. For details, visit this link.
In reply to Anonymous' comment above: Lasix is used to remove all excess fluids and not only fluid in lungs or around the heart. If her potassium is already at a 1.8 I am surprised they are choosing Lasix and not a potassium-sparing diuretic. Also, ask for a potassium supplement, because low potassium is also counter-productive for the heart's efficiency. The blood pressure is pretty low, and fluid balance is critical to proper kidney function as well as blood pressure. I wish you the best of luck.