I did that dialog successfully, with the ugly bearded bald boss-like guy and his female companion in front of the door, where you later hop to the northern parts of the Grymforge. But they ran into the room and attacked me after the fight against Nere started. Or maybe I was so confused (did not expect Nere attacking me) that I hit them with a fireball by accident.
Usually what happens is that there’s some dialogue with Nere after freeing him, during which you can confirm whether you’ll side with the mutinous duergar or side with Nere after all.
EDIT: Just to make sure you have the right duergar, you can eavesdrop on them talking about a runaway gnome before you talk to them, and the male duergar will give you a mini quest to destroy a seeing eye.