Challenge the statement that only conservatives dismiss news as fake if it disagrees with their world views and point out that forming common ground can be difficult in a forum regardless of political affiliation.
"Justine is an idiot" is no worse than "Harpo is an idiot," and it's not just conservatives who automatically dismiss news as fake if it disagrees with their world views.  I agree you can't discuss an issue if common ground can't be formed, but if it can be difficult to do this in a forum, it's hardly only the fault of conservatives.