Did you try moving your usb wire to a different USB port? I also had a problem similar to what you had and it was caused by a new usb printer that I had attached to the same computer. I unattached the printer and then the phone showed up and stayed visible after I hooked the printer back up. It has worked ever since. Weird non-reoccuring problem. Sorry I am not more help.
He can't do a restore if it's not detecting his iPhone though. I had this problem occasionally recently. My phone would not restore and kept getting kicked off and then it wouldn't recognize it. It would'nt sync either, it would have an error in the middle of it.
I restored my Computer today and re-installed all my software and it seems to be working so far. I hope it keeps it up.
I think it was a software conflict or something. You might try to restore your computer/wipe your hard drive and start from scratch. It takes a few hours, but if my phone works with my computer now, it's worth it.
I had that exact problem one time and the only fix was to set up a new user account and for some reason the new user account worked fine. It is a pain but I have gotten used to it. If you decide to use this approach let me know and I will give you more info.
Go into your services ( if on windows) and kill the service for Apple Mobile device. ( start, settings, control panel, Administrative tools, services)
reboot the PC.
Launch iTunes and then connect your phone. Wait for it to show your phone and then try to sync.
Also, Make sure that you have the latest and greatest iTunes installed. reinstall if the above does not work.