@annieo76 Welcome to Mayo Connect. Your Mom is lucky to have you watching out for her.
The description of your Mom could be a commercial for a specific category of people who get MAC and Bronchiectasis. It is called Lady Windermere Syndrome and affects active older women who are very thin with no (or minimal) history of smoking or other risk factors. It nearly always presents as a pneumonia-like condition at first, and the ground-glass opacities bear this out.
So I think you can both breathe a sigh of relief - that sharp-eyed pulmonologist was most likely correct. If she can it would be a good idea to select him to be her lung doc.
Please let me know through this discussion what you learn from the biopsy.