You might argue that there is no conflict of interest when a friend is a WITNESS. But what happens if he goes on  to become the subject of investigation, for leaks or for perjury? The subject of the many leaks from the FBI over the course of the investigation has been raised, with the possibility that Comey is responsible. If he is, not only is he guilty of that, but also of perjury, because he testified to Congress in May that he had never been the source of a leak. If that's true--or if evidence leads in that direction--what then?