Finance director overhears coworkers talking about her, gets back at them with surprising tactic: 'It was beautiful watching their faces turn from confusion to shock'

It's an interesting case, isn't it? 

The coworkers are claiming that this is now a hostile work environment. I just don't think they have much grounds to prove that! They now know that a higher-up knows exactly how they feel about her. And yet she is seemingly keeping extremely tight-lipped about the way she views them (which is great advice BTW!). 

The director comes off as being calm, cool, collected, and very professional. The duo are only panicking because they know that they were indiscrete, impolite, and irrational. They just made up incredibly impolite rumors. They said some of the meanest things they could think of. And now they know that their boss knows every single thing they said, and that she realized in real time that they were bashing her. 

I cannot even imagine being in their shoes. It must be an awful feeling! And they totally deserve everything that's coming to them. If they'd known she understood them, they wouldn't have uttered a peep. So yeah, enjoy that HR chat, and change the way you operate to be more professional in the future.