My friend has an original copy of the game and was wondering if he could play online with me if I buy the HD version on Steam.
According to the developers, the game's multiplayer mode will not be compatible with the original:
I have the original Age of Empires 2. Can I play multiplayer matches with my friends who have the new HD version?
Age of Empires 2: HD Edition uses a different multiplayer service than the original version, so cross-platform matchmaking is not possible. You will need to have the same version as your friend in order to play together.
Old question but it seems no one has mentioned Voobly.
Voobly is social gaming network, see more here
Voobly AOE2 can be installed if you have either the HD Steam version or the original CD. Voobly has it's own servers to play the game.