This is one half of two missable achievements in Act III along with:
The Swamped achievement in Knee Deep worth 19 pointsRomana persuades her cohorts to leave Gallant's shack. While with your friends inside the cabin with the gator heads on the wall, you will get the option to either convince them to LEAVE or have them STAY.
Choose STAY to get this achievement then quit to menu and reload your save to get the other one as well by telling them to LEAVE.