My customer doesnt see his site from the LAN, but He do outside of it ?¿ what´s wrong

I have a customer that has a LAN and he cant access to the website he is hosting in my server. But if He goes outside the LAN he can access perfect.
Even I can surf the website correctly, I told him that maybe there is a firewall policy in the lan restricting the access to certain sites and the sysadmins of his LAN says that everything is ok. Also, I told him to access the website through a web proxy like anonymizer, and the website worked fine through there.

So, my idea is that there is a problem in his network.

anyone has any clue to where look for in the LAN server? what thing the sysadmin should check. He can surf the internet quite well but when he try to access to any of the sites at my server he cant.

Ah!, also He did a ping to my ip a get unknown host.

I hope that you can help me with this.