My initial thoughts are this will be complex to setup as you want and you will have to change the would setup/infrastructure of your internal website for this 1 user.
You will have to:
Make the site visible from the outside world.(allow routing, domain names if neded, etc)
Disable the anoymous access (which I presume you are using at the moment) and force all the pages in your site to be logged in by the user accounts (active directory or create accounts on the server, etc)
Give the user another sepearte account.
But this way everyone will have to login to the server to get access to it.