Most cuss or "curse" words are words consisting of four letters (four letter words). Ever heard of the term, "Let's just be friends?" It is a nice way of breaking up with someone. So to him, coming from "her", the word "friend" is like a cuss word is to a lot of people. "End" is the only part of the word that he heard because he knows that the word "friend" is a word that really means the end of their relationship.