IF you know his IP and installed the right softwares, you can access his full PC and do everything you could have done while sitting on his PC
Well, although there is built in Windows Remote Desktop. I use something called RealVNC
Download it from here
http://www.realvnc.com/products/free/4.1/download.html
Once downloaded, just run the VNC server on your friend's Computer.
Right Click on Tray Icon. Click Options and VNC Password Authentication and click Configure and set your Password
Then, go to your PC and run VNC Viewer and enter your Friend's IP and click Connect. Enter the Password and you now can see your friend's PC.