CAM software has g-code converters for specific machines. Among the CAM software available is MasterCAM, EdgeCAM, and SurfCAM. All CAM software should also be able to take any part drawing and import it provided the correct file converters are installed.
In addition, I recently read that AutoCAD is supposed to have the ability to export g-codes. Unfortunately, I don't recall where I read this and don't know if this applies to other programs, such as SolidWorks, Inventor, or ProDesktop.
Your best bet for getting answers to these questions would be the CNC Tech Talk Forum. The direct link is:
http://www.websitetoolbox.com/tool/mb?username=mlynch
Registration is not required to use this forum. The people that use this forum are both advanced and novices with regard to CNC.
Also, see if the local university library has any books by Mike Lynch on CNCs. He normally does manual programming, but I'm sure the more recent books will have information about CAM systems in there.
As far as the actual learning, check with the company that your uncle chooses to buy from. I know that Fadal gives free training in Northern and Southern California, and I can't imagine that the other companies wouldn't do something similar wherever they happened to be located.
Hope that helps,
indieFan