How does meursault become involved with raymonds plans?

Raymond wants Meursault's help in plotting revenge against the girlfriend, first thinking he could have her arrested as a prostitute or having underworld friends "mark" her. Instead he wants to send her a letter to make her sorry for what she has done so she will come back to him and he can spit on her. Meursault agrees to write the letter right then, hoping it will please Raymond.