My talk with the boss helped. Post-Doc has been much kinder to Nice Coworker. The lab has been downright peaceful. She still mutters under her breath and gets a bit preachy, but it's so much better.

I think she has mental issues. Seriously.

She told Nice Coworker that she yells at her because "My work is flawless. I hold myself to a high standard and expect the same of everyone else. So I need to yell at you for you to perform to my standards."

Apparently she and another employee talk about Nice Coworker and me in their mother language. She's convinced that I understand them and have been eavesdropping on them. I'm a stereotypical monolingual American. I can pick up a few words or phrases (mostly dealing with law enforcement) but do not understand them.

It just gets better and better!