Well I am not a fan of Silver Spring. I think the area is rather sketchy and doesn't have near the amount of shops, restaurants and walkable areas. I have a friend in SS and I never feel safe catching the metro after dark in SS. SS is on the green line which is nicknamed the "Ghetto Green" since it passes thru some very suspect areas.
I grew up in Silver Spring, in the suburbs. It is a fairly diverse neighborhood. It is part of Montgomery County, which is one of the best counties for schools, but it does have its mix of wealth/low income. Has it been a while since you've visited downtown SS? It's up and coming ever since the Discovery Channel moved in. So many restaurants have opened up in the past few years.
Silver Spring is on the Red Line. Fort Totten is on red/green/yellow line. I used to take the metro home to Silver Spring at 11pm and bus to White Oak by midnight and never felt unsafe. However, I do feel wary whenever I am at the Fort Totten metro station late at night. FWIW, I am 29 years old, 5' 1", 130 lbs.
I now live in Arlington, VA, across the street from the Pentagon City Metro station. I really like it here. It is safe, near public transit, walking distance to grocery stores, malls, etc. My commute got cut in half moving here. However, I pay $783/month to rent a room in a 3BR duplex and I pay about $30-45/month in utilities.
It's really hard to tell what kind of neighborhood would be considered "best" except through trial and error.
The questions I have would be:
1. What part of DC will she be working in?
2. Will she have a car? I don't own a car, so rely on public transit for everything. If she is intending on getting a place walking distance to a metro station, the price goes up.
3. What's her lifestyle or personality like? Some people think of themselves as "city people" and wouldn't think of living outside the city proper.
4. What is her budget? Will she feel okay living with roommates?