This sounds like that when the user submits the post it's not seeing them as logged in, and thus not able to post with that user name.
My bet would be on a corrupt cookie. I would have him logout, manually delete his cookies, and try logging in again.
Make sure he isn't using your domain with and without the www interchangalbly. (for example... don't use both www.mydomain.com
There are instructions for manually deleting your cookies here
. (You'll need to login/register there at threadsdev to see that post. But it gives instructions for each operating system.)
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