not sure what you have tried, looks like the forum cut some of your post out, by anyway, the basics to try:
disable airport
plug the cat5 directly from macbook to cdj as a test, it will take about 30 seconds ut eventually the link should show on the bottom left of rekordbox for you to click on
make sure he has no symantec type crap installed like antivirus or firewall protection, if so, disable it