Why does Ben say that Biff will call Willy a fool at the end of the novel?
Ben says that Biff will call Willy a fool in Act 2, part VII of Arthur Miller's Death of a Salesman because Willy has just conveyed upon Ben (in his mind) his plan of committing suicide.
Willy is outside his house in the middle of the night planting a garden. This, he does for comfort. Not long before, he had been left alone in a restaurant by both his sons after he had an argument with Biff. Although Biff and Hap try to make amends with their mother over what happened, Linda takes Willy's side and allows Willy to go outside to comfort himself.
Willy's biggest pain is that now he sees that neither Biff nor Happy see him as a hero. He has failed at providing riches for his family, like he had planned. He also sees how his kids may not believe anymore that he is Willy Loman- the well-liked salesman.
Hence, in his plan to commit suicide, he tells Ben that he is not only doing it to get the insurance money for his family, but also as a way to demonstrate that he IS indeed well-liked.
Ben, that funeral will be massive! They'll come from Maine, Massachusetts, Vermont, New Hampshire! All the old-timers with the strange license plates -- that boy will be thunderstruck, Ben, because he never realized -- I am known! Rhode Island, New York, New Jersey -- I am known, Ben, and he'll see it with his eyes once and for all
Ben then retorts that not only may Willy not get the insurance money (since it will be a suicide and not a natural death), but also that Biff, especially, will think of his father as a coward and a fool. Shortly after, Ben dissappears from the scene, and Willy still considers his plan.