A traceroute reports the path that a request takes from a computer to its final destination.
Assuming the use of windows, one can run a traceroute by opening a command prompt and typing:
Or www.yourdomain.com as the case may be.
The command prompt can be found on Windows by going into the Start menu and typing "cmd" in the search box or going to the All Programs -> Accessories folder.
What the person should be looking for is for the final entry on the traceroute output to be the destination web server. If the traceroute fails to make it, then there is a problem along the line. Subsequent requests may take different routes to the destination server, so multiple tests should be performed. This is an excellent article that I think will explain things better than I can:
Also, since the person is having consistent problems, maybe they could try accessing the site using a proxy just to see if the problems continue or not. A proxy like hidemyass.com. If they have problems accessing directly and no problems accessing via proxy, then that can help isolate it to their ISP's network--or not. It depends on where the problem lies.