Get them to grab the ISO, and a modified MF.dll file, Including the Project Cartographer Launcher. With the installation, just skip the activation (Activate Later), THEN once the install is finished copy+paste the MF.dll file into Halo 2's dir.
https://github.com/FishPhd/H2V-Online-Launcher/releases
All found at the bottom of this page. If you want the ISO, check out the current (But dated) installation guide for Halo 2. There should be a link to a working ISO there.
This should work, I've tried it before
I might also do an installation guide. Like the Halo online on I did a while back:
http://unitedfalconclan.roflforum.net/t1067-halo-online
Lemme know what you think.