Yes. That should work.
What I would do is start the download so that Steam recognizes it as a partially installed game, close Steam, copy the files, and restart Steam...I believe that has worked for friends at a LAN party, I can't remember for sure though.
Also, Source games share resources I believe, so downloading HL2, and then ep1, and then ep2 should save you bandwidth, you'll probably only have to download 40% of the game or so.
LEFT4DEAD411.COM
Bet with disolitude: Left4Dead will have a higher Metacritic rating than Project Origin, 3 months after the second game's release. (hasn't been 3 months but it looks like I won :-p )