i live in Melrose, which is a very nice, residential city. close to a lot of things, and still on the Orange line (Oak Grove). IIRC, TJX's offices are in the Back Bay, near where I work. So the Orange is the line to be on. we pay $1250/mo for a big 2 bedroom in a 2 family. very quiet. we paid the same for a 3 bedroom in Medford that was half the size and on a busy street.
but if your friend wants to live "in the city" cambridge, south end, somerville, brighton, and allston might be better options. rents a bit higher, but if he finds a roommate or 2, it should be managable.
i also have a friend who lives in Brookline for $2100/mo 3 bedroom with 2 other roommates. but he's on the Green line which is SOOOOO SLOW.
also tell your friend to check out Craig's List to apartments. and some of the college housing sites. he might get a chance to room with a nice college co-ed!