Well, here are the steps I would take in order of probably working/simplicity:
1. Just assume the guy knows his **** and double-click the files to open them in quicktime player. 99.9% of mpeg's will just work. I give about 75% chance the avi's will just work
2. If Quicktime gives an error, choose "Continue" to open the movie with no video. hit command-i or command-j (depending on what version of quicktime you have. doing both will not hurt) to show the movie's properties. Look for where it says "format:"
2-A. If you see IV32, IV42, IV50 or IV-anything, that means Indeo Video of some version (the versions are not backwards- or forwards-compatible). Go to <a href="http://www.apple.com" target="_blank">www.apple.com</a> and search for Indeo. These codecs were created by Intel, but are no longer under development or supported in any way, which is why Apple has to host them. Download all the different Indeo codecs you can find, and follow the included instructions to install them. They only work in OS 9 or below.
2-B. If you see DivX, DIV3, DIV4, DIV5, DX50, or MP43, go to <a href="http://www.versiontracker.com" target="_blank">www.versiontracker.com</a> and search for divx. OS X has many different options, but I'm assuming this is an OS 9 only situation (8.6 might or might not work, anything below that and forget about divx). You'll probably find Quicktime codecs from <a href="http://www.divx.com" target="_blank">www.divx.com</a> and <a href="http://www.3ivx.com." target="_blank">www.3ivx.com.</a> Either will work. I use the 3ivx one. Just pick one and download it and run the installer and follow the directions. Both websites have forums just like this one where you can get support
2-C. If you see MP42, go to homepage.mac.com/collectivesw/ and look for the wmaaudio.sit file and download it. double-click it to create the wmaaudio file and put that in your Extensions Folder in the System Folder and restart (these directions are probably on the website, but you never know). There is a component for OS X, too, search for MSMPEG4v2 on versiontracker for it.
2-D. If you see MPG4 or MP41, you will need OS X. search for MSMPEG4v1 on versiontracker and follow the directions. If it's MPG4 you will have to change it to MP41 by one of the following routes: (1) download DivX Tool from versiontracker. I think it lets you change the FourCC. Change it to MP41. or (2) download a Hex Editor like HexEdit and open the movie in it. Search for the second occurance of MPG4 and change it to MP41 and save the file.
2-E. You might see some other things, like HuffYUV, that are Windows-only. But they are uncommon, and mostly only used as intermediate formats anyway, so don't worry about them
3. If you double-click the file and quicktime opens, but doesn't open the movie, it's either corrupted or not really an AVI/MPEG. It might be an ASF or WMV, in which case you can download Windows Media Player (versiontracker.com again). If WMP doesn't want to open the file, try changing the name to end with .asf or .wmv (copy the file from the cd to change the name)