There are a few things that could cause this problem, and I encountered them a lot when I was working tech support for both iPod and iTunes.
The first thing to do is log into or make another account on the computer and see if it happens there. Only use a few files so you aren't re-adding a lot of files over and over again. If nothing gets erased upon a restart, the iTunes information, or other information, in his profile might be corrupted. The best fix is to move anything in his user folder somewhere else and use the new account, or he can go into some of the folders (I don't remember which, though he may find them on the Apple support site) and check the permissions on iTunes preference files. If it does get erased, try the following in the new account.
One is where his default folder is. If he adds it all, but it isn't moved into his default folder, or the default folder is pointed elsewhere, iTunes sometimes doesn't like it. Make sure his files are in a backup. That's always important when dealing with iTunes, and particularly on Vista. In iTunes, go to the Edit menu, select Preferences... and go to the Advanced tab to check the default folder. Make sure it's pointing to where you want it, then have the other two boxes under it checked. It appeases the iTunes gods. Well, not really, but iTunes might be more cooperative. Re-add some of the content, make sure everything looks good, then close iTunes and restart.
The restart normally isn't necessary, but it's to check for the other possible problem. If upon restart and reopening iTunes, the content is gone again, go back into the Advanced tab in preferences. If your settings didn't stay, try changing the default folder to something else, readd what you had added before, boxes checked, and restart again. Once again, if the content isn't there, check the default folder. If it's not what you set it to, then it isn't saving changes for some reason. Exit iTunes and look into the iTunes program folder. Check the permissions on the folder and of some of the files. Some of them may be read-only and not allowing iTunes to save to them, losing the information it has about content. He can also check in the user folder (check the Apple support site to make sure you know which files you're looking for) and see if there are read-only permissions set for the preferences files there. That would indicate that for whatever reason either iTunes or Vista is creating those files read-only or with the wrong permissions and so iTunes isn't saving everything properly.
The last thing that you can try is completely eliminating iTunes and reinstalling from a fresh download. You will want to check the Apple support site for that, because there are a lot of files that may not be erased when iTunes is uninstalled, and they have a complete guide for making sure it has been completely removed from your system.
When all else fails, call Apple. They have access to a full database of possible solutions that you may not have found on the site, or may not be available to the general public.