It might be becausethe game is not on a CD-ROM, it's on a DVD-ROM and the older computer may not have a DVD drive. If that's the case then he won't be able to play the game on the older computer as they don't do it in CD-ROM form.
If he's desperate to play it, then he can try taking the copy he's got back to the shop or selling it and then downloading it through Steam, an online game delivery service.
Check out
http://www.steampowered.com and you can download the client package and go from there. He will have to buy the game again though which may cause problems, but it's the only way you can play it on a computer that doesn't have a DVD player on it