Anyhow, within a day of her first getting her Mac she had managed to install something called Mackeeper (I think?). It took some digging to find out that it was not some sort of malware protection but actually was the malware.
It probably got installed due to the fact that the MacKeeper people plaster ads for MacKeeper all over the place (presumably only if your web browser's user-agent indicates you're on a Mac, though). These ads strongly suggest that installing MacKeeper will make your Mac more better in every possible way and that you should do download and install it right now because reasons.
Complete bunk, of course, but it can work on the right type of impressionable mind (i.e. "the computer said I should do this, so I'd better do it" -- not making the distinction between what the OS is recommending and what a third-party ad is recommending). I get a phone call from my mom every 6 months or so asking me if she should install MacKeeper or not -- I'm grateful that she knows to ask about it and not just blindly install it.