Sorry to tell you but there's nothing we can do about it. Once it has been transferred there's no way for you to reverse it back. It's your fault, maybe you signed up from airdrop or something. Heads up, if you're going to use MEW make sure that you don't put all your ETH and altcoins in the same wallet.
all my security measures are on
Well, unfortunately there's not much you can do (except laying a criminal complaint against unknown but it most likely won't help).
However tbh I don't understand why anyone would store 45.79836 ETH without using a hardware-wallet (which costs about 59).
As you might be infected by malware I'd recommend creating a backup of your important files, wiping and reinstalling your OS and changing all passwords -- if you've got more wallets you should transfer your funds to a new wallet - (a hardware-wallet would be the best choice).
I'm also confused why he don't use 0.5 ETH buy a hardware wallet to store his 45 ETH, you too brave store crypto assets on online wallet, computer include antivirus software doesn't mean anything, antivirus can't 100% protect your computer won't infected virus, unfortunately transactions on the ethereum blockchain cannot be reversed once sent, so don't expect you can get it back.