She had heard rumors and knew that the Idahos did something to make the Harkonnens angry. There she saw an advantage, and turned them in to get a reward. She was a guest of the Idahos on the day that Harkonnen troops came to their house and took Duncan's parents, himself, his uncle and two cousins.
However, she did not receive the reward she had hoped for. She received no money and no promotion. When she approached the Harkonnens to ask for some reward, they ordered her not to come back. She then decided to escape the planet.
Janess Milam had known that Duncan was taken to the Forest Guard Station to be hunted for sport by Glossu Rabban, and she tried to find him and turn him over to the House Atreides, the Harkonnens' greatest enemies. She had parked her flitter which was discovered by Duncan.
There she arranged with Renno, the Second mate of a cargo craft, that Idaho could board the ship and escape the planet.