I wonder if she's a closet smoker? Having been around family members with COPD just about all my life I know they can't walk very far without having coughing fits. They start out with the hack that gets progressively worse over time and walking aggravates it.
I have emphysema BECAUSE of that (quit in 2012 ... too late) ... and I completely forgot about that.
That could be a contributor, but her gait indicates a muscular/skeletal condition.