You can reconfigure Proxy Server to only allow a specific workstation to use the internet by changing the LAT to only include the IP of that specified workstation.
To do this, firstly give the chosen workstation a fixed IP address, and exclude it from the DHCP scope. Then go into the Proxy MMC, drill down to the Web Proxy and go to properties. Select the “Local Address Table” button, then type in the workstations IP address for the From and To sections and add it to the LAT. Remove all other entries in the LAT, give the workstation the fixed IP address mentioned above, and restart proxy.
You may also want to change the permissions to remove all access control, therefore making the workstations IP address the only limit to users surfing the web.