This is story related and cannot be missed.
Spoilers below if needed-
*** Spoiler - click to reveal ***For reference, after returning from Guarma and moving camp to the north west of the map - dealing with a cave full of Murfree Brood members,
will be brought back to the gang and admit they were a traitor, the achievement will pop after this scene
As pointed out in the comments by Shadow XBL. The Dutch Mission that triggers the events in the spoiler above appears on the map at the same time as a mission for Sadie, do the Sadie Mission first. It is missable and counts towards another achievement when done with it's second part a little later.