This achievement is on Case 4 and it is MISSABLE.
After you obtain a search warrant for Veronica's room and you find the victims missing earring on her bed, go back and use it as evidence when you are interrogating her. She will confess to sleeping with Will Rice at the time of the murder. Now go and talk to Will Rice to see if he backs up her statement, and he will admit it's the truth. This achievement will unlock after your conversation has finished with him.
For a walkthrough of this case and a guaranteed way of getting this achievement please use this: walkthrough (Thanks to Yelll o for this great walkthrough)