Fellow traveller - Wikipedia
Additionally: https://en.wikipedia.org/wiki/Mitl%C3%A4ufer
The German term Mitläufer (plural: Mitläufer) was used after World War II by the denazification hearings in West Germany to refer to people who were not charged with Nazi crimes but whose involvement with the Nazis was considered significant to an extent that they could not be exonerated for the crimes of the Nazi regime.