A 412 error? That's a new one.
Googling it shows it has nothing to do with your site. It's a "VPN" error, and has to do with his internet connection. One site says this:
* Poor network connection. Packet delay or loss can force the VPN session to drop.
* Hardware failure in a network interface card or network device.
* A firewall or ACL (Access Control List) is blocking required protocols or ports:
o For PPTP, IP protocol 47 and TCP port 1723 are required
o For IPSec, IP protocol 51 and UDP port 500 are required
Sounds like wither his router (if he has one) or maybe the card in his computer has an issue, or alternatively, he needs to contact his ISP.