A 3 day programming test?!
I guess if you were desperate for a job that may be acceptable, but no way would most businesses even attempt such a thing.
I think that they will just send him a test and want his results within three days. That would be exactly the same as my test at my current employer was. I got some code and had make it compile as well as fix logical errors and improve the performance. In total I probably spent about 5-6 hours on it before sending it back. They simply gave me three days to do it in case I had other things to do during that time.
As for preparation, I don't know if you can really prepare if you don't know how the test will look like. You should make sure that all your tools are working and you know how to use them.