If Scott's surmise is correct, the backend mail server does not support threaded view, and it sounds like that one user has tried to select view as threads. Have the affected user go to Settings->Folders, select the Inbox, and change the View mode from Threads to List.
I can't recall that there is a config setting to disable threaded view, but if the process above fixes the user's Inbox, you'll want to at least warn your users not to select viewing as threads.
