Well, there may be no DRM, but I'm pretty sure it's against the terms of service to make a copy for someone who has not paid for the game. That's one of the reasons for the duo pack - two games for the price of one.
If you think your friend would be interested in getting the game, you could show him part of it in single-player, then tell him "if you had a copy, you could be controlling the other guy, making decisions, wandering around..." and if he was interested (but not enough to buying it for full price), but you bought a duo-pack you might even be able to get him to fork over $20 for a second key.
