I'm afraid the physical disk will try to link the copy to the Steam account currently logged. What you could try doing is copy the bulk of your installed data over his Steam installation folder (basically just the biggest folder containing all the game's packed data, excluding the executables and dlls under the root install folder, perhaps via an external hard drive).
Your friend would have to buy a copy for himself, start downloading the game, close Steam down and then copy your content over his folder. If it's clever enough, when he restarts Steam back and resumes the download, it should detect the data as valid and avoid him the task of getting everything, downloading just the executables and generating the DRM APIs for his account.
A repair through Steam's client would follow, to ensure everything's fine.
I hope it's clear that this is just an hypothesis; I never tested this nor guarantee it works. I also hope it's clear that he won't be able to play and that you'll risk getting locked out of your account if he doesn't buy himself a copy, as well.