Philip Mahue is the ex-friend of high school sociopath Harry Baker. Philip cut himself off from Harry when he got even more disturbed and obsessed with death, witnessing Harry victimize a cat firsthand.
Philip was found at his school huffing in a bathroom stall and detectives threatened him with reporting his addiction. He revealed what he could, including circumstantial evidence that made detectives suspect more of two murders they were following him around for: he saw one of Harry's stashes of skulls from animals he slaughtered, which were found with a large hunting knife the detectives suspected of being the murder weapon.
Harry was revealed to be innocent and released. (SVU: "Prodigy")