If you are the network administrator, just setup a VPN either by using a SonicWall Firewall or through the Domain Controller some other router and access the QuickBooks by using the Remote Desktop Connection on a local machine in your client's office. This would be the Safest and the Fastest way to access QuickBooks Remotely.

Or you can host your QuickBooks which would be a more expensive route.

Good luck,

AlcompCo