Tough situation!
I second the suggestion of password-protecting your computer. This includes using password protection with your screen saver, in case you forget to log out. Do this on both your laptop and your desktop.
You can also enable parental controls, either directly on your machine or through your ISP. Parental controls should block at least some of the porn sites, and you can explicitly deny access to anything it doesn't catch.
Then, I'd talk to your friend, and show her your browser's history files as proof.
I wouldn't confront him directly - but be prepared that he may confront
you when he finds his access has been cut off.
Take care.
(btw, I like ariesrising's idea too
)