Just checking if there is anyway to avoid this slightly stressful situation again, I booked an elderly relative (wheelchair requested) on UA ticket, seat selection was extra on AC, which we did not pay for, this already wasn't a cheap ticket. When they checked in there were no more seats available and SBY was indicated on final segment BP. The agent at the airport said they would not allow passenger to be left behind as their policy is to not deny boarding to wheelchair passengers. In the end they cleared standby list, but the situation wasn't ideal.