Mavericks is OS X 10.9, which is 2, soon to be 3 major versions behind. May want to advise them to take the free update to El Captain (10.11) and the soon to be released Sierra (10.12). Mavericks will likely fall off the supported list for receiving security updates when Sierra comes out in a month or two.

Also, double check the security settings. The malware (not a virus) got in somehow via a user initiated action. Sadly a lot of bad advice exists out there to disable Gatekeeper, which basically lets malware run free on the platform. https://support.apple.com/en-us/HT202491 has more info on this system.