Assuming you can still boot into windows --
First Download the Magical JellyBean Keyfinder ( http://www.magicaljellybean.com/keyfinder/
) For this usage the free version is fine. Install it and run to find the key and print a copy for your record.
Second - GO to Digital River (Ms's approved site for downloading Iso images of win 7) -- ( http://www.mydigitallife.info/official-windows-7-sp1-iso-from-digital-river/
) and download the correct versin of windows 7 to use for reinstalling windows.
THird - Burn the iso image to Disk or create a bootable USB thumbdrive with the image for installation.
You now have the iso and the product key so are ready to reinstall (Personally with the price of HDDs I'd buy a fresh HDD and do the install on it to make sure everything functions properly and that the install activates and keep the old HDD intact so it can be installed as a slave drive to get the files he still needs from it instead of reformatting the old HDD - that way if you forget to make a backup of something it is still on the old HDD.