He knew that Charlotte was tormenting Anna and chose to say nothing. Valentin and Nina saw the Metro Court security footage where Charlotte entered Anna's hotel room. Red ink was found on Anna's things, and a warning in lipstick was written on the mirror. Nina deleted the footage at Valentin's request.
The paint in Charlotte's backpack matched the "murderer" graffiti paint on Anna's front door. The paint was in the backpack that Charlotte had with her when she broke into Anna's place (formerly Maxie's pad), where Anna shot Charlotte in the dark as she rifled through the steamer trunk that had the WSB evidence file.
Anna was out of her mind paranoid about a past WSB mission, and was linking everything to that, when everything but the arson was Charlotte's doing, and Valentine said nothing.
Victor sent a letter and tarot cards to Charlotte at her boarding school, saying Anna was the enemy and she must help her poor papa. After Charlotte was shot,Valentin ransacked her room and found the letter.