Just a follow up to say thanks for all of the advice
Since my metamour's mental health is...erratic and she has been a hot mess lately, I decided not to bring it up to her- I'm usually the first one she attacks when she's feeling messy. I did tell our shared partner in as non-judgemental a way as possible. Just a "X said something about Y the other day, wasn't sure if I should say anything, but it was a bit weird"- then relayed the story and my response to it. He was surprised and agreed that it would be a problematic statement if it was said, but as I was, thought it was strange that I couldn't get any context for the story when it was told to me, as the incident when this was said occurred during a game of Citizens Against Humanity (for those who don't know, it's a card game where awful, awful things get said simply because you have to work with the awful things written on the cards). I left it up to him to deal with her about it- I've told him before that her personality issues are his problem to deal with, not mine- so he can deal with this too!