Not download binaries of the internet and execute them.

The mail program the user used was probably written by Apple, one of the biggest companies in the world. And Mail is probably one of the most hardened and scrutinized against attacks applications in the world.

If the user is afraid that Apple mail is faulty or malicious, then installing software from tiny companies is suicide out of fear of death.

Have you forgotten the silent remote code execution vulnerabilities in iMessage, also one of the most hardened and scrutinized applications made by one of the biggest and richest companies in the world.