As CheekyChops indicates, there is no requirement for an employer to advertise a job internally, externally or at all. (He can just give the job to his best mate if he wants to).
When I was teaching, the first headteacher I worked for had a policy (which he openly stated) that he would never promote anyone internally even if they were the best teacher in the country. On his way to the top, he'd never spent more than three years working in any one school and he made it very clear that he believed that everyone should be looking elsewhere for a job within a couple of years of joining the school.
Chris