The president should fire him and explain exactly why, taking much of the text of his explanation from this article - i.e. this is not how we do things in the US.
If Trump does not start cleaning out the infestation in the DOJ immediately after this report the country is heading for serious trouble. This secret police apparatus is undermining our democracy in insidious ways. Allowing the Deep State Cabal to get away with this subversion will only encourage them to do even more of the same. It must be stopped NOW.
DOJ is an executive agency under the control of the president. With the release of the Horowitz report (even if watered down) Trump will the justification and political cover to clean house. If not now, then when?