Safair overbooked flight and left Travellers at the airport

My brother was supposed to fly on an Safair flight to Durban yesterday (14 may 2018 at 16h50), he checked in online and the got to the airport (Or Tambo) just after 3pm and went straight to checkin counter for them to print him a boarding pass and was told the boarding time is at 16h20, he waited, by 16h35 he(and other travelers) was at the boarding gate and was denied to board the flight as they were told they were late. And they were forced to book the flight for the following day. This is very unfair because they were denied boarding 15mins before departure time. We want Safair to refund us the money for the flight because we suspect that they overbooked the flight and we forced to return people at the boarding gate who were on time.