Sorry it took me a bit to get to this post,
lasa. I've been working away from the computer for the last couple of weeks.
Depending on what version of OS X (as your Dad is using IE, I suspect it is an earlier version of OS X) Preview may or may not play animated GIFs (it is supposed to.)
Unfortunately Mail, the default mail application, while it is developing into a fine program still has no option to automatically play animated GIFs.
So here are the solutions. If your Dad is using a version of Preview before Tiger (10.4.x), have him launch Preview. From the menu bar (#1) View > Customize Toolbar... which brings up the drop down window with all the toolbar options. Drag the play button (#2) to the toolbar (#3) and you can play animations. If he has Tiger, there is a bug in Preview which does not play the animations automatically the way they are supposed to.
A second work around is to save the GIF to your desktop and drag it onto a browser window. It will play automatically. Actually you can save it anywhere, but I tend to just drag files sent in Mail to my desktop.
A third option is to use a different mail application. Thunderbird is an excellent mail app which allows for the playing of animated GIfs automatically, and the creation of simple HTML mail similar to Outlook Express (BG colors and that sort of thing). Mail app is very limited in those areas. I and many others have put in suggestions that we would like to have GIFs play in Mail but still no change, though it does now render HTML mail extremely accurately.
http://www.mozilla.org/products/thunderbird/