This is wholly incorrect. You can look long and hard to find a legal or contract provision suggesting that UA has an obligation to rebook on another carrier, whether in *A or not.
But, the larger issue is that OP has not supplied correct details and this makes advice hard. There is no UA 2016 today. The 08:30 EWR-DCA is anticipated to arrive 32 minutes early and the closest flight to IAD is at 10:25 with a projected arrival 3 minutes early. Beyond that. important to know whether the friends were booked on the same ticket (as opposed to PNR). If two tickets, they may be SOL entirely, although UA may rebook as a courtesy on the next UA flight with availability (whenever that is).
If on the same ticket, UA will reroute in this order: UA, *A, other carriers. Other carriers is really hard if there is anything remotely within *A, with a preference for LH because it is a JV. Best to research the options and have them in hand rather than asking UA. Give consideration to IAD-LHR via other UA hubs such as ORD and via YUL/YYZ on AC. Finally, UA at IAD may well have a good idea of availability on later flights and your friends may well find themselves on a standby list very likely to clear.