intentional drop (he may not know the rule out in his judgment it wasn't intentional), then the force is in play. The runner at first no longer has a right to first so long as the batter is not yet out. If the batter gets out first, then the base runner can lay claim to any bag and there is no force.
So as long as they tagged the runner before tagging the bag, and the ump did not rule an intentional drop, then it's a double play.