When the Chit-Chat window pops up after watching enough of the important videos, answer yes to both prompts. This will cause Sarah to log off and walk across the road to meet up with SB.
I'm not sure exactly which videos cause the Chit-Chat window to pop up, but it may be related to terms like arrest, murder, mother, dead, etc. A full transcript can be found here:
https://www.appunwrapper.com/2015/07/09/her-story-full-text-...This achievement is necessary to continue to unlock the following two achievements: