When is a Muslim not a Muslim?

I thought I knew the answer to that question from what I had learnt here but after a recent conversation with a Muslim friend of a friend I am now not sure. Is it simply a matter of complying with the 5 pillars of Islam? The man in question accepted and rejected any verse of the Qu’ran, any hadith and any aspect of the sunnah as fitted with his personal beliefs. He wouldn’t even commit himself to agreeing that every verse of the Qu’ran was the word of God rather than the words of Muhammad. I found myself suggesting (from what I have learnt here) that with such beliefs he could not legitimately call himself Muslim but when he asked why not I was stumped.