Both achievements (this and Shadow Ally) focus on the same condition - side with a faction to complete chapter 2 upon reaching 15,000gp, then complete their quests before departing for Brynnlaw to complete Chapter 3. There is a catch though.
The easiest of the two is Shadow Ally, as this achievement is hard to miss since you can ally with the Shadows Thieves at any time. Friend of Darkness is different, as you must ally with Bodhi when asked to meet her upon reaching 15,000gp - you can't stall joining up with her, as you only are given the choice there and then to join her.
However, it is possible to get both achievements in a single playthrough by saving before reaching 15,000gp, as the meeting reqest with Bodhi is triggered upon reaching this amount. Since Friend of Darkness is missable because of condition mentioned above, I recommend doing this first, even if you are not intending to ally with her during the playthrough.