The only thing I can think of is that if his name (say "John Smith") is identical to a John Smith poster that has already been posting under that name and with a password (say "XXXX"), then the new John Smith can't use that name and his own password (say YYYY). It prevents somebody from having an imposter make posts under another person's name (unless the imposter also guesses the original "XXXX" password too).

Has he tried changing his name to "John X. Smith" or "Johnny Smith" plus his own password?

It's a long shot, but it's the only thing I can come up with.