That's a horrible disease even when it's not so advanced. You and your family must be in so much pain at the moment. I believe that non-smoking women have about a 1-in-75 chance of getting lung cancer during their lives; natural background radiation (especially from radon gas) is generally accepted as the biggest single cause. Like your mother, about half of these non-small cell cases are discovered at advanced stages.
I'm sure you'll spend a while looking into the details, but here's a quick precis: The prognosis at that (worst possible) stage is unfortunately quite poor, with five-year survival rates too low to bear thinking about. The cancer is considered both incurable and inoperable. (In a few situations, some surgery is done to relieve symptoms.) Multi-drug chemotherapy (the old-fashioned kind) and radiation can reduce symptoms (a benefit not to be lightly discarded), but its net impact on median survival is typically measured in single-digit months, so quite a few patients (especially those with other health problems) reject the distressing treatments and simply plan for palliative care. I do wish I had better news to share.
I am so sorry to hear that your mother is dealing with this disease. Please accept my sympathy.