I know there are other solutions available here but I don't feel any of them properly explain what pops this achievements. Near the end of the case, you will get a call saying that Veronica lied about when she got off work. When you get this call, go to the interrogation room and question her about this. She will specifically say "I couldn't have been there, I was in bed with Will." To get her to say this, I believe you have to confront her with the cell phone pictures from Manny's phone but that's not the only thing that triggers this achievo like some of the other solutions seem to be saying. When you hear this line, go back to Brass and ask to speak with Will Rice. You will get a line of dialogue asking basically to confirm her story and he will agree. This is when the achievo pops. I hope this helps anyone who was confused.