The two girls, Miss Forsythe and her friend Letta can be regarded as a plot device. They help precipitate the stormy climax of the play, the final, fateful Loman family row.
Biff and Willy fall out once again in the restaurant over Biff’s failed meeting with Bill Oliver; unable to take any more, Biff leaves abruptly, just after the girls arrive. Seeing how things have gone so badly wrong with Willy, Happy, rather than lose face in front of the girls, denies any connection with him.
No, that’s not my father. He’s just a guy.(Act II)
Happy, then, denies that Willy is his father at all, simply because he wants to keep up appearances in public and go on impressing the girls. For their sake, he leaves Willy alone in the restaurant; although he subsequently claims that he only did this in order to go after Biff, he clearly also has a good time with the girls in mind: ‘honey, we’re going to paint this town!’ He is an inveterate womaniser.
Happy’s behaviour in denying his father and chasing after the girls causes Linda to finally snap when he and Biff return home:
Did you have to go to women tonight? You and your lousy rotten whores! (Act II)
In fact Linda is so angry that she orders them both out of the house. This makes Biff decide to have it out with Willy, to try and explain himself once and for all, about how he is not able to live up to Willy’s ideals. This ultimately leads to Willy’s suicide.