Macbeth says "a friend" the main reason is because he don't want
Banquo to suspect him for killing king Duncan in the future. The
dramatic irony is used for telling that Macbeth is going to kill the
king while Banquo still dont know Macbeth's intention to kill the
king. Moreover, at the beginning of the we know that Banquo is
Macbeth's friend, so replying that answer is just very natural.