D'Vines, D'Vinos, Schneiders, U Street Liquour are the 4 stores I've been to that have great selection. If she can make it just outside of DC, my first choice would definitely be Westover Market in Virginia. Unbelievable selection
The Whole Foods on P St has a great selection as well, as does Conn. Ave. Wine & Liquor in Dupont Circle. Also check out Arrowine in Arlington as well.
It's a 15-20 minute drive, but unfortunately not too accessible by metro. If she can't make it there, the suggestions above would certainly suffice