That he knew to be shady or with dhady characters.
Like what he has done but I doubt he is lilly white clean.
Without evidence, we really don’t know.
And in a larger sense, it doesn’t matter, since his testimony corroborates with the laptop evidence.
One thing we do know for sure is that he certainly cares out his reputation, and he is certainly someone with great personal courage to be willing to go public with the information.