Not really sure what the question is, but from what I can tell its not exactly a hardware issue provided you install IE on the laptop in some way or you can access the app from other browsers on the Mac - e.g. Safari, Firefox or Chrome. If you insist on IE well Macbook Airs are Intel processor based so you just figure out how to get Windows onto it. For example if you don't want to dual boot Windows from a bootcamp partition, then
Install Windows within a Virtual machine - e.g. VMWare fusion / Parallels or Virtualbox amongst others and browse from there. You can "merge" the Win/Mac desktops if you want so it feels almost seamless - i.e. IE use almost feels like a Mac app. You'll need some extra RAM for this so I hope user bought a 4GB Macbook.
One of the most common apps run on WINE on Mac is likely IE, so figure out how to install IE on Wine and you're set. Not too scary but some command line skills might be good. Check out something like: http://davidbaumgold.com/tutorials/wine-mac/ for a fair start.
There's also a related commercial version called CrossOver - see www.codeweavers.com
If your user just wants to run IE on Mac maybe this ... http://www.macupdate.com/app/mac/33367/winebottler is an elegant one piece solution.
If you want to avoid all this and its on a LAN then find an unused Windows PC and setup remote desktop support to it and use the Mac Win RDP client and use the remote PC to do the IE browsing.
I mean I run a Macbook Air on Snow Leopard and happily use Firefox as a web browser even for those sites that are very IE specific in 99% of cases. I run QV 10 personal edition in a Win 7 virtual machine on it, so can't comment on QV specific Ajax from Mac browsers but I have worked with developers who created websites using many Ajax features and I've tested them just fine on the same laptop from Firefox and Safari.
Not sure which IE plugin you mean, but on Firefox 7 mac I can see IE view lite 1.35 which looks promising. You just need to give it a path to the IE install - could be bit of fun to do on a Mac but try see if you share/map the folder containing Iexplore.exe (assuming its available while running a background Windows VM on the Mac) and see how that works from the IE view prefs. There's also a plugin called IE netrenderer 0.97 - may do something similar.
Have lots of fun,
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