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Stuffit won't do iso files. Extract-xiso (command line) should work though:
http://sourceforge.net/projects/extract-xiso

If you want a GUI for it, you can try Xdisc:
http://www.command-tab.com/2006/12/29/xdisc-mac-xbox-iso-utility/
Or he can double-click on the .iso to mount it on his Desktop. ISO files are disc image files. If the OP's .iso is the image of a DVD, then DVD Player will play it. If his .iso is the image of an audio CD, then iTunes will play it. If it is the image of a data disc, then the files may be accessed directly using MacOS X applications which are compatible with their document formats.
 
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