1. Which "wrong course" is the book referring to? What the DFed person was DFed for or the "wrong course" of the current JW talking to them? I've always assumed is was the former.
The wrong course is the rebellious course that resulted in the unrepentant attitude culminating in disfellowshipping. A family member, who continue to assciates with the DF'd is likely to be influenced by the unreasonable thinking of the DF'd one, and so therefore needs to be encouraged to maintain a view to treat the rebellious one as if they were a Tax Collector. Any association wil be putting your spiritual health at risk.
2. What's your experience with this issue: JWs getting caught talking to DFed close relatives?
They usually get grassed up by some spiteful member of the congregation and the Elders will dissuade them from associating with the dF'd one, pointing out the dangers and reminding them how close we are to the end of this system of things, that we wouldn't want to displease our loving father Jehovah etc.
The last time I shepherded someone in this situation, they only really wanted to visit their grandchild, I told them to be discreet!