From The Grey House, Book One:
If he told Vincent he wanted to investigate Natalia’s disappearance, then he’d have to tell the man how he knew her. He would have to tell her he had slept with her and would probably have to divulge any and all information he had about her. Charlie really didn’t feel he had the right to do that. Everything she told him had been in confidence, and although Vincent might consider his silence a betrayal, talking would be a betrayal to Natalia. He already betrayed her once by trying to turn her into a werewolf.