With that time frame I would be hesitant to leave the airport and even to leave the secure area - getting back through security can take longer than you want it to at times.
Once they arrive at 5:30 am, it will take them until at least 6 to deboard and get out to the main terminal and Marta station. Even if they have to wait for a train, that is the beginning of commuting time Marta so runs more trains, so they could be in downtown Atlanta by 7 am. Not much is open at that hour. They could walk around Centennial Park, but the Aquarium, World of Coke & CNN will be closed. Walk around and look at the buildings.
If their flight leaves at 11 am, they should to be back at the Marta Station by 8:30-9 am, so only 1.5-2 hours in downtown Atlanta.
I agree with the other suggestion to check out the other concourses. Highly suggest walking to get some exercise. There is a terrific exhibit of stone carved Zimbabwe art in the walkway between the Main Terminal/Tgates and Concourse A. It really is worth checking out.
You can get breakfast at A, B or E, all of which have great options.
On your way to breakfast walk from A to B to see the new exhibit on the history of Atlanta. It's ok, and it is on your way back from the Zimbabwe Art.
Not sure if you can get into it without an international ticket, but they just opened the new International Concourse F last week. It is quite a nice new building worth checking out if you can get in. It also has some interesting art.