> Or - hear me out - the partner was embarrassed to verbalize that she's competing with the friend's engagement ring, and therefore created a less embarrassing, post-hoc rationalization as to why she wants a $40k ring too. Here's a thought experiment - had the friend gotten a $6k ring, would she have asked for a ring closer to $6k or still gone with $40k, by some intuition?

Marginally better, but still greatly concerning that the person you're supposed to trust most is too embarrassed to communicate openly imo.