Zoom in IE6/7 is a very strange beast - when you zoom in/out all affected elements receive display: block and may change their position. I actually am reproducing it in IE7 with several RadMenu controls on the page with at least half of them affected. Unfortunately there is no fix for the broken zoom in old IE browsers (not that the zoom in IE8 is much better, but at least it works okay most of the time). Can you try if the problem persists after your client removes his browser zoom?
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