The closest Metro Station to the Ellipse, where the Christmas tree is located would be Farragut West, on either the Orange or Blue Line. From there, it s just a walk straight down 17th St to the Ellipse. With that being said though, it s still probably a good half mile walk.
No, not Smithsonian. Unfortunately, there is no stop very close. If you look on Google maps, you will see that Federal Triangle is the closest, but Farragut West and McPherson Sq are also close. They are all around a half mile walk to the Ellipse. If she really has trouble walking that distance, you should spend a few dollars and take a cab from the metro stop you choose. If you really need to save money, you can take a bus: see wmata.com.
Orange/Blue Line: SmithsonianAs I remember, it s still a good walk--as you have to cross the Mall.Here s an interactive map of the Metro, so you can see where the stop is:http://www.wmata.com/rail/maps/map.cfm