I recently had a knee replacement and could not stop traveling for business. Delta handles wheelchair requests very well. You can have her met by a helpful wheelchair attendant at every point of the trip. It really does not make much of a difference which connection point although ATL is more difficult than the others.
Just make sure to order up the chair for her when you book the ticket and whoever takes her to the airport tell the first Delta person met that a wheelchair is needed for this passenger. Also, tell her that since she is in first, get her 1A so she can look out the window when each flight docks so she can see if the chairs are already there, as they usually are. If they are not, she can just sit comfortably until they arrive.