This is unlocked by killing Navarre with her killswitch. That's done by hacking both hers and Manderley's PCs during the return to UNATCO HQ. As you are ascending from the basement, you get the killphrase on level 2 and Navarre appears on level 1. She doesn't attack on sight, so you can use the killphrase during the dialogue.
This unlocks the following achievement too: