Issues connecting to VPN

Hi, i work for an ISP in Australia, our clients use PPPoE to connect (no modem at all, they simply use a router or their computer to connect straight to a CAT5 cable which goes to a server etc etc) the particular client has a Macbook Pro 15 inch running leopard, unfortunately i am not sure what hardware the MBP is running. They are having issues connecting to a VPN based in the USA with our connection... when going through a router the issue exists, the router was by passed and the same issue occurred... we do not block any ports at all over our network, so everything should work without issue... i am not sure on what kind of server the customer is trying to connect to at all either, but all i know is that when she uses a USB 3G service, the VPN works fine. I have a feeling that the router was blocking the connection, but i am not sure why straight PPPoE from the MBP did not allow them to connect... could there be a problem with connecting to VPN networks whilst being connected to a PPPoE session? If anyone can answer this question it would be much appreciated.