When there's mutual distrust then it both makes sense that she would want a recording kept on file, but it is also a break in trust to not have him aware that she was recording. No idea how it is in Korea, but there are laws in the states that a recording has to be LEGALLY recorded for it to be worth anything. Nonconsensual recordings are clearly not legally binding.
That being said, the other course of action with an 'illegal' recording is to publicize it, whether actually in public via media or to higher ups within the company. I'm sure there are people who want him out of that position more than she does.