Hey,
At my first practice, we would never make an appointment without the patients say so. If an appointment had to be moved, we would either try calling them if it was short notice, or sending them a letter if it was a few weeks out to say that the appointment had been cancelled. We never just rebooked them without their say so.
However, at my new practice, if we need to move an appointment, we just send them a letter saying 'we have moved your appointment to blah blah. Please call us if this is inconvenient'. If they don't show up, we just assume they haven't got the letter but we don't mark it against them.
It sounds as though your friend has missed quite a few appointments. We usually give a patient 3 chances before they have to be moved to a different dentist (still in the same practice but down the 'hierarchy' as it were. If they are completely hopeless, then we refuse to see them and I believe we are in our right to.