He would be an Involuatarily denied boarding. He would be entitled to much more that the fare difference, although I am not sure. But I have read other threads where yoy get money, vouchers etc.... Read the contract of carriage, get to the airport and see what happens. A lot of people in first class do not show up. I buy first last minute tickets and end up not going all the time.

But if they put you back in coach, you will be compensated. As a Z fare, you would be trumped by D and Fs.