Let him now sue Apple for releasing a fake app into its store - because if he was naive to download it, how naive and stupid are those who allowed it to appear there in the first place?
I am pretty sure that Apple is protected in this regard. They have an extensive legal team that has thought of everything a single mind can think of. If we went through their App Store terms carefully, I am sure we would find points saying that end-users accept all the responsibility of the software they download and install, and that Apple can't be held accountable even if malicious apps are hosted on their platform.
How difficult is it to check the official website of a piece of software and assess whether it is a fake or legit app when it comes to those who approve the addition of apps to official stores?
Not very difficult. But you could ask Jimmy the same question. They both failed miserably.