Do iMacs even need that stuff?
There is indeed a such thing as malware on the Mac, and so antivirus software is just as important there as it is on Windows.
What makes her think that she has malware ?
Though Andy's question is valid, your earlier description, in another thread, of the way SWMBO "just clicks on things and installs them", is pretty much a textbook description of someone at high risk for malware infection.
Unfortunately, I don't know much about Mac antivirus, so I'll let someone else suggest good products in that area.