The location for this achievement has been changed from the 360 version. Near the end of the mission, you'll be heading down a tunnel with some lower areas on the left and right. Head down into this lower area on the right-side (if you're facing the direction to progress the mission) and you'll reach a door leading to some stairs that go down to a holding cell. Either kill or knock out the guard standing over the prisoners before he kills them to gain a Moral Point and unlock this achievement. I would suggest knocking him out so you can get the Invisible Man achievement as well.
I've created a full Enlightened playthrough video, and during the Frontline mission I point out collectibles and Moral Points as well as showing where this achievement takes place and the path I prefer to take when going for Invisible Man while still gaining said collectibles and Moral Points.
You can see the full mission with all items of interest and collectibles at the 1:01:42
mark in the video.