Captain Holt: You said there was no mole. You said you knew everything about these people.
Jake: Well, it turns out I don't. I've learned some truly horrible things tonight. You were right and I was wrong.
Captain Holt: Goodness. Boyle is sleeping with Gina?
Jake: What? How could you possibly know that?
Captain Holt: "You were right and I was wrong". You must have been shaken to your core to say that to me. Boyle is your most trusted friend, so he has to be involved. Your use of the word horrible leads me to believe that the matter was sexual in nature, given your obvious immaturity.
Jake: I've had sex.
Captain Holt: And, of course, you would be most upset if Boyle were to have slept with someone you knew from your childhood. Thus, the solve: Gina and Charles.