" What are you trying to expose your friend to,the single player or the multi player? If it's the former, you could just give him your disk and tell him to sign in as a guest. If it's the latter, load him your account (only if you really trust him) and let him play around for a bit. "
From JohnDM: You can also buy the entire Penumbra Collection for $5 with any donation to the Humble Indie Bundle. Only works through the Frictional Games store (so no Steam codes) but you can get the whole Penumbra series for only $5 on any platform.