Atlantic Station (for shopping) is in Midtown, and can be accessed via MARTA by getting off at the Arts Center Station and taking the free shuttle bus across the bridge to Atlantic Station. She would take a northbound train from downtown for this.
Also at the Arts Center MARTA stop is the Woodruff Arts Center, including the High Museum of Art, the Atlanta Symphony and theatre performances. Google Woodruff Arts Center for various schedules.
Not sure how she would access this on MARTA (by bus), but the new Ponce City Market (in Midtown) is in the process of opening and it is very cool. A few restaurants and shops are open now with more opening each day. But the real sight to see is the magnificent building - Google Ponce City Market for info.