There are huge numbers of people who got on with life after the terrible days of war and chose to let sleeping dogs lie. Perhaps this is one of those cases. Obviously, many thousands (or more) saw Hitler as he went about his role.
My mother has vivid memories of seeing the aftermath of an air raid shelter taking a direct hit in the war. She has had a morbid fear of fire ever since.
Why not simply say to her... "I hear from (insert name here) that you actually saw Hitler yourself... What was happening then?" or something like that. She will either share her memory or dismiss the subject. I doubt that the actual occasion would form a traumatic memory for her.