It's perfectly normal in Windows XP for the pc to go back to a login screen after a screen saver. However, the user should not be automatically logged off. Check to see if, when it next happens, there is a notice underneath his name that says he is already logged in. If there is, you're fine. Off the top of my head, I don't remember how to turn off this "feature", but it should be somewhere in the control panel, and is probably accessible by right-clicking the desktop and going to the tab where you normally change screensavers.
If he is truly logged out, then your problem is different.