How to help my friend install Microsoft Office for Mac 2008

Hello, I have a friend in North Carolina whose mom recently got a MacBook Pro running OS X Mountian Lion (10.8), he will be coming on to join this discussion after I post it. He needs Microsoft Office for his mom and I just happened to have purchase 3 licenses (two of which I have already utilized). While talking to him over a Facetime call, I walked him through the steps of getting the free office for mac 2008 package from download.com (which seemed kind of sketchy). While the download actually looked legitimate, we ran the installer and it gave some kind of weird error message that said something like "Could not be installed onto Macintosh HD because the Mac OS X is not on that parition" (I'm paraphrasing my friend will contribute the actual message). Could someone please walk us through an easier (and preferably more secure) process? We just need to get to the point where I can give him my last license key. Thanks!

YraYra