Bus etiquette

A girl sitting two rows in front of me decided to mind the seat next to her for her friend. As this bus is full and a lot of people got on before her friend, she had to tell them that it was reserved because, understandably, they saw a vacant seat and wanted to sit.
I felt that her friend should have been on the bus earlier if he'd wanted a seat and it should be first in, first served when it comes to bus seating (except for injured, disabled, elderly or pregnant people, of course). I was hoping that one of the passengers would sit there anyway and say that he should have arrived earlier at the bus stop if he'd wanted a seat.
Is it possible to reserve seats on public transport for friends? Especially when some of those the girl rejected looked to be full-fare passengers and this girl and her friend are more than likely on concession fares.