On a similar note, Reb Yaakov Kamenetsky explains that the reason the mitzvah of tochacha ends when the person gets upset and says to stop (ad k’dei nezifa), is that since the entire mitzva is to help the person out, if he will not accept, at that point, the mitzvah does not apply anymore. (The friend may, of course, get another chance to respectfully speak to him another time, but for now, he is supposed to stop.)