Columbia is a good place to look.
She can take the Commuter Bus (or drive) from Columbia to Walter Reed painlessly. There are even express buses that connect the Columbia Mall to Silver Spring (most of the way to Walter Reed).
http://www.mtamaryland.com/services/...ineForWeb2.pdf
http://www.mtamaryland.com/services/...ineForWeb2.pdf
Rent anywhere in Maryland near DC is more than $600-800 a month (and if it is that, it would be in an unsafe neighborhood). Perhaps she can pay the difference or share costs with a roomate.
I used to live in Columbia and I paid $1300 a month for a one bedroom apartment with a den and a view ~ but it was a really nice complex.
I've heard there are nice, less expensive rentals in Columbia, but she will probably find that the going rate is around $900-1000 a month for a small apartment.
There are also apartments in Silver Spring/DC, but once again, the safe ones can be even pricier than in Columbia.