When Charley is introduced in the play the stage directions are as follows:
In all he says, despite what he says, there is pity, and, now trepidation.
It seems that Charley understands and pities Willy. In their initial conversations in the play, it is Willy who insults Charley. Charley does not trade insults with Willy.
But Willy exaperates Charley; and Charley becomes quite frustrated when Willy asks to borrow money from him but will not accept Charley's offer of a job.
When the hell are you going to grow up?
But if you look carefully at his lines, Charley's advice to Willy, though actually harsh is really quite sound. He understands Willy better than Willy understands himself.
The only thing you got in this world is what you can sell.
At the end of the play, Charley seems to have nothing but empathy for Willy:
Nobody dast blame this man.
Charley is a good friend to Willy. He would have been an even better friend had Willy let him.