iChat problem

I have a mac pro and a macbookair and no problem with video ichat. I'm in the record industry managing various Artists and all works fine - ichat with record company, artists, agents etc except one of my artists has a new macbook and can't ichat - always says communication error after about 15 secs. He can text chat fine and I can see a green light on my machines to say he is online but both voice and video ichats don't work. His broadband connection seems strong using a BT Homehub router. Any ideas - really urgently need to get him working - Thankss