You're reading it wrong
They don't charge an under occupation fee.
They're saying if you want to change your holiday- this is the heading for this bit of the terms and conditions.
So ifyou change your holiday and less people go the individual cost for the each person may go up
So if 10 people.go- then due to the number the price per person is £400. Total cost- £4000
But if you want to change your holiday after booking and less people go the individual cost per person may go up.
So if 9 people go the cost will be £420 per person and total cost will be £3780.
So although the cost per person has gone up as per terms I would assume the total cost should remain the same or less.
I wouldn't call this a cancellation charge.
Even if all of you don't turn up they won't charge you extra for this.- it just means you won't get a refund.
So I think if he just didn't turn up. All it means is you won't get a refund for him. Not that they will charge ypu.
I mean planes are usually overbooking all the time. And expcet people to miss their flight. They arent charging those people a no show fee. Same with a hotel. Some overbook. If people.dont turn up. They aren't charging people no show fees.