Aloha!
It is possible that your mother might have seen AH in the border region of Germany and Austria, as he had his favourite residence just outside Berchtesgaden ? I'm sure he might have made trips locally both privately and for functions. Maybe it was before WW2 really got going and AH was more relaxed in his movements before becoming paranoid after the attempt on his life.
It was a moment in modern history for your mother to have seen AH, but as in all these sightings (including the present) it probably wasn't more than a passing excitement (either positive or negative), much like I see the Queen close up at the Epsom Derby each year. Time seems to give these encounters a magical sheen, perhaps because they cannot be repeated.
The participants of the turmoil of WW2, and other conflicts are often reticent to discuss their experiences, putting it behind them as the best course of action to get on with their lives. I would definitely ask a parent/uncle/auntie about different aspects of their experience, they can choose to say no and that also should be respected.