only one person can be logged into the same steam account at once
wrong, sorry, but if one person starts playing, and the other goes to play, all they have to do is re-enter their name and password into steam and they can also play. trust me, i've done it before, and it works. the only thing you cant do is play in the same server. Me and a friend tried this a month ago and we were both playing the same game (Counter Strike) on the same account in different servers.
besides all you would have to do for your friend to get HL2 is download HL2 on your account and get the computer into ofline mode and then use it from there. the computer with the extra copy of HL2 wouldnt even need an internet connection after it is downloaded. So I see no reason why the same copy of HL2 cant be put on multiple computer sysytems.