Ask Bernie not to endorse warhawk Hillary. Tell him to graciously congratulate her on her victory, but short of outright endorsement. He should certainly denounce authoritarian Trump; but ask him not to advise or persuade his supporters to support Hillary. Let his supporters democratically decide for themselves how to handle the election and how to vote.
Why is this important?
Bernie's main purpose was to revolutionize America and the Democratic Party. This is the main goal of his supporters too. His supporters need him to continue to lead them toward this goal of democratizing USA and the Dems. It will take a while because after November we will have two Right-Wing parties. If he endorses Hillary this will all have been for naught.