My friend who has forth stage breast cancer has fluid around her lungs, tumers inside her lungs and is tired. could oxygen therapy help her at all?

If her oxygen is low: Oxygen level can be easily tested and so i would do that first. If it is low, supplemental oxygen can be quite helpful. If too much fluid, she may need to get treatment to remove the excess fluid, otherwise oxygen may not get to the lungs to cross to the blood where she needs it...

Yes: As dr. Bessmer stated it can be checked to see if oxygen is low by use a pulse oximeter (you just place it on the finger at get a reading) it should be in the mid to high 90's. Otherwise oxygen bottles or generators can be ordered by your doctor. Sometimes the o2 level is good resting but walking it gets real low so it can be tested when walking as well.