Since DCA will get her 39 more EQMs each way, it's clearly the better choice!

Seriously though, if she's unfamiliar with the DC area, I would recommend she fly into DCA. When I go to unfamiliar places I like keep things simple at the final destination and taking advantage of the airline's support for as much of the journey as I can. So in this case, she'll have the airline to help her with any problems in ORD and they have multiple daily flights from ORD-DCA. Once on the ground, DCA provides a much shorter and simpler journey to her final destination. Otherwise, it's probably a toss up in terms of time, convenience and price, especially if she actually going to downtown DC.

As someone who lives in DC, I'm learned the ins and outs of get to/from each airport so for me other factors come into play. Even though I live about 15min from DCA often fly from IAD despite the greater hassles of getting there.

All that said, here's my analysis for your friend:

Total travel time - This seems to be a toss-up.
The IAD non-stop wins airport to airport saving 1.5 hours according to United's estimates. However, she will use up every minute of that at IAD getting from gate to terminal and then to downtown DC. The most cost effective and relatively efficient means of getting to DC from IAD that I know of is the 5A Express Metrobus. It leaves IAD every 30-40min and for $6 it will take you to the subway at Rosslyn VA (near Georgetown) or L'Enfant Plaza in DC (near National Mall). This bus only makes two other stops and takes about 40 min to Rosslyn and 1hr to L'Enfant Plaza. Then you have subway to wherever she's ultimately going, probably 10 min.

The DCA flight would take 1.5 hours longer but it would bring her within 10 min and $2.35 (by subway) of the DC border and probably another 10 min to her final destination.

Ease of getting from airport/gate to downtown - DCA wins this hands down. It has direct walking access to DC's clean and generally efficient subway system, proximity to downtown DC (5 miles vs 30 miles) and gates close to main terminal (no mobile lounge-ML or train from terminal to gate). IAD has walk to ML or train, ride ML/train to terminal, terminal to 5A bus, then 5A bus to subway.

Price - IAD usually has an advantage with the 5A bus since airfares are usually cheaper than DCA (e.g., $169 vs $241 on 2/28 on UA). Obviously, taking a cab or shuttle from IAD reduces this differential.

Other Intangibles - DCA wins on views. Most often this flight will come down the Potomac from the north given beautiful views of monuments Washington flying into DCA (window seat on left side of plane). Also there great views across the river from the DCA terminal and on the subway to DC (yellow line). DCA also is a bit less busy and the proportion of experienced flyers if probably higher. (This might help on the return flight).

Traffic from IAD. On the 5A bus this usually isn't as big a factor as it might seem given the DC area's traffic problems. It uses two limited access roads for much of its route, somewhat limiting exposure to the congestion.

BTW, here's a link to the DC Metro System for bus and subway information.

Good luck on the decision and tell your friend to check out the Cherry Blossoms if they're still around when she' here.