You do not need os9 at all. If you have the original Halo combat evolved CD, then you're supposed to be able to install by double clicking the install icon. It should look like a red downward pointing arrow, entitled Halo Installer. You MUST have os 10.2.8 or better, panther is recommended...
It's the installer that you need buddy, not os9. The installer app should be on the CD, and it might say something about this in the manual. If not check on the halo official website.
He might have been talking about the installer app. I think halo uses aladin installer, and the installer package may have been trying to run itself using his os9 version, because it was called the same, and didn't have the osx version