How do remove Stuxnet from an imac

It's extremely unlikely he actually has Stuxnet on his Mac.

This worm was written to seek out and infect hardware specific to operating centrifugal machines in the purifying of uranium.

Once the code was discovered (and stupidly) made public, the Flashback trojan was designed around the Stuxnet code. That was back in 2012. Flashback has been patched against on the Mac since then.

In short, he doesn't have it.

Stuxnet was broken and defeated over a decade ago.

Your friend does not have stuxnet on his Mac.

I'm 77.

Stuxnet was designed to speed up nuclear fuel refinement devices until the self destructed; unless he happens to have those nuclear level centrifuges on his Mac, he doesn't have Stuxnet.

However, if you want to prove it to him, get Malwarebytes and have it run a scan.