The first thing Hap does in the restaurant that represents his character is lying about what Biff does. Hap says that he's a successful cattle rancher. This is typical of Hap's personality we can see from the Requiem. Hap wants to believe the best in people, and he believes his own lies. In Hap's head, Biff really could be a successful cattle rancher, just as we see in the requiem, he really believes in Willy's life as anything but a failure.
Secondly, Hap wants to leave Willy alone and go with the two woman and Biff, he ends up telling them that Willy is not his father. He wants to do this for the same reason as above, his father's just been fired, and Hap is unable to deal with this. To deny that Willy is his father, means that Hap won't have to deal with anything, which is how he survives the whole play.