Definitely Ronald Reagan National Airport would be the easiest and most convenient since she'd only have to take the Yellow line metro and be there in less than 20 minutes.
From Dulles in VA it's a bit more complicated because it would involve walking for half a mile, taking a bus, and then the metro, which would take about 1 hour and a half.
From BWI in MD the distance is longer, but it would take less time. A little bit more than an hour. I actually work very close to BWI, so if she arrives in the afternoon I'd have no problem carpooling and taking her to the venue. I also have a VIP package. We'd have to arrive a bit earlier for the exhibit, though.