Thanks. I could examine some typical log files and come up with some suggestions regarding which files may contain useful information, but I would be doing more or less the same as you could do on your own.
Open and peruse the human-readable files using your favorite text editor; perhaps one or more of them will have logged some obvious problems. Fixing them is another story altogether.
Wireless connection problems are very common. Refer your client to If your Mac isn't connecting to the internet over Wi-Fi - Apple Support. The Summary page in that document is the most useful, but it's also very generic, due to the multitude of vagaries endemic to wireless communications in general.
The only entry in that Summary page should be "Review Wi-Fi Best Practices". That's it. The appearance of anything else requires investigation.