Air India - BOARDING DENIED

Today (**/*/**) My father was supposed to travel in Air India from Kolkata(CCU) to Dibrugarh(DIB) flight was at *:** a.m.He reached airport on time and received boarding pass as well from Air India Counter, check-in counter was a bit crowded so he had to stand in the queue for some time and when he finally reached at the gate he was simply denied boarding by the unprofessional staffs of Air India saying the gate was closed and not possible to reopen even after that my father asked them to simply reschedule the flight or refund the amount and they denied all the options saying since the booking was not done directly from their website so it is not their responsibility to refund any amount. Whose fault was this??