I like Spokes in Alexandria (Quaker Lane store) but mainly for their mechanics. I've purchased bikes from them but not as a returning/new biker. Steer clear of Revolution Cycles in Clarendon.
Suggest your friend make the drive to College Park and go to Proteus.
I think NYBiker likes the Bike Lane -- it's in Burke, not too far from Alexandria (she can get on Little River Turnpike and head west). Also, I've been in Conte's in Ballston and got a good vibe from the folks who work there.