Use 'qua' to clarify the capacity or role someone has in a situation. She participated in the discussion qua expert, not merely as an observer.
Pro Tip 2/3
Strict Separation
'Qua' emphasizes a distinct aspect of an individual or thing, separating roles sharply. Qua author, his words carry weight; qua celebrity, much less so.
Pro Tip 3/3
Academic Flavor
'Qua' often appears in academic or formal discussions to sound precise. Qua historian, she provides a unique perspective on this issue.