Faked "his" death at the Battle of Hogwarts. Ran away to the USA to work at Salem. Voice trained (for femininity and American accent), adopted a new name, and got on Magic HRT "to really sell the disguise".
Isekai setup: The magical American government picks her to be the first witch on the moon after they invent a very long-range teleport ritual and she invents a very versatile environmental condition protection potion. They send her to the dark side with a magic tent full of supplies; she ends up somewhere rather farther afield.