It may be related to the phone network he's using. I had exactly the same problem when I first set up an account.
Gree appears to have blanket bans in place on cellular phone networks. So to this day, I can not access the forum if my phone is using the Vodafone IP address, and it appears that I am banned, but it works fine using my home broadband.
99.9% of my time here is via iPad on the 3 network, so it's not a universal ban on mobile networks. I'd agree a blanket ban on the provider is the likely cause though.