Reddit reviews A Practical Guide to UNIX for Mac OS X Users

One of the problems with the "virus question" is that people still have a very outdated view of how infection works. We got very used to how propagation worked in the 90s and early 00s. Often it boils down to an all or nothing argument.

In addition, the objection to being 8-10% of the population being enough to slip by is a correct one. It's not enough and it never has been. However, falling back on an "antivirus" as a way to protect yourself is not necessarily the correct approach either.
The best thing a modern Mac user can do to protect their system is learn about how to secure UNIX systems. This is key to protecting your Mac.

For starters, take a look at these books:

'A Practical Guide to UNIX for Mac OS X Users': Amazon | Google

'Practical Unix and Internet Security': Amazon | Google

It's not wrong to be suspicious of files you download. However, these books will help orient you and learn the philosophy and practice of securing your Mac.


EDIT: I guess I didn't directly provide an answer to your question, so here goes: You said spam was being sent from your mother's email account. One thing to check is whether her account is IMAP based. Check the headers on the spam mail to try and determine it's origin. Often email spam can boil down to a few things:

1. Someone is spoofing the email address and it only appears like your mom's
   
   
2. Your mom's password has been compromised and someone is using her email account to send mail.
   
   In either event, they are probably not using your mom's mac as the source to send emails at all. It's a good idea to check but the header on the message in question should confirm your suspicions. In any case, have her change her passwords asap if she hasn't already. Good luck!