My oldest daughter lives in Arlington. If your friend is bored and doesn't mind interacting with strangers, I suspect she'd be willing to entertain her for an evening, given enough advance notice.
I didn't read carefully all the replies, so apologies if I'm repeating what's already been posted. Arlington National Cemetery and the Tomb of the Unknown Soldier should be seen at least once in a lifetime...especially if she's staying in Arlington. It's a very moving experience, and I never tire of going there - and I've been a number of times. One can either walk the grounds or ride in trolleys.
I would disagree with the poster who said the National Mall is rundown and depressing. It's true they are reseeding the grounds, but there are great views of historical buildings and monuments. And, if she's there in December, the National Christmas Tree is pretty cool.
The Holocaust Museum is also quite moving, and well worth an afternoon. It's located right off the Mall. I would agree with those who say to not bother with the White House tour. You really don't get to see much, and I think it's a bit disappointing. However, nearly every time I'm in DC, which is often, I walk by the White House on Pennsylvania Avenue. It's iconic, and looks totally different from what you see on TV.
Another cool place is the Old Post Office. You can ascend the tower in an elevator and get great views of the surroundings. Some people think the view from here beats that from the Washington Monument. If she's near the Mall, the National Museum of the American Indian has hands-down the best cafeteria. It's great food, and not terribly expensive.
Finally, since she'll have a car, if she has any interest in aviation and space, Udvar-Hazy is a must-see. I could spend a week there. She will definitely want to utilize the Metro (subway) for getting around DC. It's next to impossible, not to mention expensive, to park anywhere downtown or near the tourist sites. The Metro is clean, safe, convenient and goes nearly everywhere a tourist would want to go except Georgetown.