Be sure to put it on a CD to give to him, then again, I don't know how else you could give it to him. Has he run adaware, or any spyware removal tool, to see if any spyware has changed his registry. I had a problem where spyware wouldn't allow me to connect to the internet, but never where spyware would stop me from logging in. I don't know of any registry scanner's you could download.
What operating system is he using?
If he uses XP, did he download SP2, and it is causing conflicts.
He could also try going to start - run - and then type in MSCONFIG and looking at his startup settings.
I don't know how this works, because I found it on the internet, but if he has the XP cd he can go to Start - Run - and then type sfc /scannow and it will scan and replace corrupted Windows Files. As I say, I don't know how well this works if it works at all.
What happens if he starts in Safe Mode and doesn't log on as the Administrator. Instead he logs on to his username/account. Does it still freeze on him. Or he could create a new user in Safe Mode, and then restart in normal mode and try to log on as that new user and see if it works. If it works he could then transfer the files from one user to the next and delete the old user that froze the machine. Or he could remove the password from his account and see if it works.
Here is somebody with the same problem running a XP machine -
http://forums.techguy.org/archive/index.php/t-211613.html
When he goes into MSCONFIG he should look at these two tab's as the most important probably, but he can also look at the others. He should try to start using the diagnostic startup and then after booting with that switch back to normal.
If none of this helps here are some Google Searches you can look at -
http://www.google.com/search?hl=en&q=%2Bxp+%2Bcomputer+freezes+on+logon
http://www.google.com/search?hl=en&q=%2Bxp+%2Bcomputer+freezes+on+login&spell=1
Microsoft might have some other stuff on the MSCONFIG settings, or you could do a search.