How reasonable is this concern?
Very reasonable, if your employer is a CA. Not at all reasonable if your employer sells hubcaps. Need more info.
How can this sort of malware be prevented?
Educate employees. (But your next question shows that you already know this.)
Is there a way to educate employees...?
Yes. Employees are not algorithms. That's why we employ them instead of just computers.
This current reality is that people have started to rely on having their smartphones...
Yes, if you want effective employees, you should allow them to use their brains, as well as extensions that make them more effective.
Do you have any questions that lack obvious answers--perhaps something worth discussing in a forum?