An AVI file is a "container" so there is no assurance how it was encoded. Since there are now hundreds of possible encodings on AVI files you can never be sure it will play without some work.
Did you try VLC Player on your iPhone yet?
Short primer at http://osxdaily.com/2014/08/03/watch-mkv-avi-video-ipad-iphone-vlc/
Hi, I've been sent an avi file video and don't have a computer. It's been sent to my yahoo email account but when I click it just appears as a grey screen with play icon in the middle but won't open. Any ideas what I can do. Thanks

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