(1) All of John's friends say they know someone who smoked for 40 years. (2) John does not know anyone like that (3) John is not unique among his friends in that respect, i.e. John is not the only one among his friends who does not know anyone like that.
Since premise (3) contradicts premise (1), it is impossible that ALL friends know someone like that and SOME of his friends do not know anyone like that, we can conclude that SOME - at least one - of his friends are not telling the truth.
Let me know if this makes sense and if you have any further questions.