All the UA flights from IAD to BOS (whether mainline or express), operate out of C or D gates at this time. From the time your parents exit the first flight until they board the connecting flight, they will be the mid-field C/D concourse. To reiterate some of the previous information: when they exit the arriving flight they will follow the purple TRANSFER signs to a down escalator at the east end of the C Concourse. On the ground level there, they will go through Immigration, Baggage Claim, Customs, Baggage Re-check, TSA security. After TSA, up an escalator into the C Concourse at C7. Then walk to the gate for their Boston flight.