You can move the wallet to linux
Install bitcoin-qt in linux ( ubuntu ) and move the existing wallet.dat to that
This makes the wallet more secure than windows
If he used it on Windows before, it would be pretty much useless. Your wallet.dat might be exposed to all kinds of malware on Windows. Therefore, if you move it to Linux, the attacker would have your key and can steal your BTC.
Do you realize how difficult you are making this? How many Windows based people would know how to find an Ubuntu install and burn a copy. Of those, how many would be able to get the OS installed and run bitcoin-qt in under 1 day? Telling Windows based people to move to Linux is not helpful. People who are familiar with Linux can do this, people who are young and have time can do this. With the exception of miners, I would say Windows based bitcoin-qt easily accounts for 85% of all the offline wallets.
All this talk of malware is quite useless. If you still have the coins in your wallet, use this thread
https://bitcointalk.org/index.php?board=37.0 and pick a wallet on an OS that you're comfortable using. Switching to a new OS while trying to secure your wallet = lost coins. Once you have a secured wallet, simply move your coins from the unsecured wallet to the secured wallet - problem solved.