Advice wanted on possible GPL abuse

[matthew]

Hi,

I'm investigating (on behalf of myself, I am not the copyright holder) a case
of what I believe to be GPL abuse. An individual has produced code
modifications for the (GPLed) phpBB forum software, but is selling them under
a non-free, non-GPL license. He claims that this is acceptable since he only
links to phpBB via phpBB's defined functions.

A link to his claims:   
http://www.phpbb.com/phpBB/viewtopic.php?p=896232#896232

I'd be grateful for people's opinions on the matter, please CC me as I'm not
subscribed.

Thanks.