I believe he's referring to IE 8, not the OS. I assume he has a 64bit OS, and somehow downgraded to 32bit for IE. Now, he wants to go back to 64bit IE. On that note, I have no idea, lol.
If that's the case, installing the 64 bit version of IE automatically installs the 32 bit version also, so he's half way there. He just needs to reinstall the 64 bit version of IE.
There are very few 64 bit plugins/addons for 64 bit browsers so don't expect a lot of things to work.