I think you have to set the network region he's using to use hairpinning.
If you don't the softphone may try to communicate directly with one of the conference members. Since IP packets can only have one destination, it may prevent the other users from hearing him.
If you force hairpinning, it causes all of the packets to be delivered to the PBX before they're distributed to callers.
It's a bit more intensive on the system, but it's makes sure the users can hear all parts of a conversation.
I would also make sure that his PC network settings are correct, and that it has a proper network gateway.
Carpe dialem! (Seize the line!)