Everyone has mentioned the main tourist destinations, but here are some off the beaten path ideas – she might find one that relates to her interests.
If she is into politics or history at all, she might enjoy touring the Capitol building. It is right downtown, and it has a museum inside with displays on the history of Atlanta and Georgia.
If she likes literature, she might enjoy visiting the Margaret Mitchell house at Peachtree and 10th. She can take MARTA and get off near there then walk.
If she likes money and finance, she might enjoy the amazing little money museum inside the Federal Reserve Building. It is right across the street from the Margaret Mitchell House and has samples of counterfeit money, and robots that move stacks of bills around, and a machine that shreds worn out bills. Visitors get a free bag of shredded money.
If she likes law, she might like the small museum at the State Bar of Georgia, which is downtown. It has some art drawings from famous cases, and some exhibits on law.