Just like the best kitchen alligator trap, the best anti-virus for OS X is the one that has caught something. In other words, they are all the same; none of them catch any viruses.
You are confused about these terms...
-- virus: app that attaches itself to another app and can self-replicate.
-- worm: app that is unattached, self-replicating, self-sending (via network or attached disk).
-- Trojan horse: app that appears to be desirable, but is not.
-- hacker: someone (a real, live person) who uses computer knowledge to get past your network firewall and login.
Your mother did not get hacked. She got included in a group email list chain letter. Those grow until there may be hundreds of thousands of email addresses on them. A few of these 100,000 people is a jerk who sells the list to a spammer. The spammer uses a "spoof" of your mother's email address. That is like getting a post card that says it came from Santa Clause advertising a big sale at Macy's. You know it isn't really from Santa, and those emails sent to people on your mother's list did not really come from within her account. Ask her to look at her "Sent" folder of her email account. She will see that none of those emails were "Sent" from her account by anyone. They just look like it to the people who received the emails. This type of spam is the most effective, because most of us already know "Don't open email from strangers", so people trust your mother, and open the email.
If she wants to stay off the spam lists, she has to convince her friends to use BCC. Good luck with that. I send out a plea for BCC to all my contacts once a year, and it does no good at all. They just don't get it.
Links below are for the free OS X anti-virus apps. They do just as well as the ones you buy, which also don't catch any alligators.