Red Line
Rockville covers a fairly large area along & past the Rockville Pike
The Washington Metro Red Line follows this road (which is an
extension of Wisconsin Blvd as it leaves the District) for much
of it going into Rockville with several (but not enough) stops
She might consider proximity to 1 of these Metro Stations
The Red Line has a stop at Union Station & connects to
the Yellow Line at Gallery Place to get to DCA (very handy)