To what legal authority does he bring this information?
By doing "nothing" he at least keeps them a little bit off balance, wondering what he looking at, until a more favorable Administration returns to power. If he pulls the trigger and makes public allegations now, then - to use Al Gore's infamous phrase - there is "no controlling legal authority" that is going to do anything about it...other than taking an eventual revenge on Durham and his family and his allies.
You have a very valid point!