Preparation - Reunion
Upon reaching the trailer park, you can proceed along the left side path and spot a woman holding a baseball outside of her trailer. Talk to her about her husband. Now, with the same character, go to the grey trailer next door (with the sex noises), and knock on the door to talk to Cletus. Then, return to the wife to tell her what you saw. She'll take care of it with her baseball bat and the achievement unlocks.
This achievement can be seen in the video guide at 7:37:NOTE:
Achievements are only available to owners of the game. Players using the "Free Trial" friend pass will NOT unlock achievements.