This is called Hidden City Ticketing and is against all domestic carriers' rules except for Southwest. United would have the right to cancel your DD return trip, go back and charge her for the trip she actually took or both. Of course if there is checked luggage getting off at DC is impossible.
From United's COC (
http://content.united.com/ual/asset/UA_COC09Mar11.pdf):
WHERE A TICKET IS PURCHASED AND USED IN VIOLATION OF THESE
RULES OR ANY FARE RULE (INCLUDING, BUT NOT LIMITED TO, HIDDEN CITIES
TICKETING, POINT BEYOND TICKETING, THROWAWAY TICKETING, BACK-TO-BACK
TICKETING, OR SPECULATIVE/ABUSIVE TICKETING), UA HAS THE RIGHT IN ITS
SOLE DISCRETION TO TAKE ALL ACTIONS PERMITTED BY LAW, INCLUDING BUT NOT
LIMITED TO, THE FOLLOWING:
(1) INVALIDATE THE TICKET(S);
(2) CANCEL ANY REMAINING PORTION OF THE PASSENGER'S ITINERARY;
(3) CONFISCATE ANY UNUSED FLIGHT COUPONS;
(4) REFUSE TO BOARD THE PASSENGER AND TO CARRY THE PASSENGER'S
BAGGAGE, UNLESS THE DIFFERENCE BETWEEN THE FARE PAID AND THE
FARE FOR TRANSPORTATION USED IS COLLECTED PRIOR TO BOARDING;
(5) ASSESS THE PASSENGER FOR THE ACTUAL VALUE OF THE TICKET WHICH
SHALL BE THE DIFFERENCE BETWEEN THE FARE APPLICABLE TO
THE PASSENGER'S ACTUAL ITINERARY AND THE FARE ACTUALLY PAID;
(6) DELETE MILES FROM THE PASSENGER'S FREQUENT FLYER ACCOUNT;
(7) REVOKE THE PASSENGER'S ELITE STATUS, IF ANY, IN THE MILEAGE PLUS
PROGRAM, TERMINATE THE PASSENGER'S PARTICIPATION IN THE MILEAGE
PLUS PROGRAM, OR TAKE ANY OTHER ACTION PERMITTED BY THE MILEAGE
PLUS PROGRAM RULES;
AND/OR
(8) TAKE LEGAL ACTION WITH RESPECT TO THE PASSENGER/AGENT.