Can an aurora australis be bright enough to illuminate someone's face?
If your eyes are fully adapted to the darkness you will be able to see a persons face in the light from the auroras.

By fully adapted to darkness means sitting in a totally dark room for 20+ minutes before you start looking at the night sky. The time you will need to adapt depends on your age.

Any bright light will destroy your night vision. As we get older our night vision will get poorer so young people will see better at night than older people do.