Speaking as a girl without even the least bit of interest in diamonds, "If you give up the thing you want, you can get a thing you don't want that's TWICE AS BIG" is just in no way a selling point, especially as long as her association with those stones is that they're fakes. You can try to make the moissanite seem cooler, or the lab diamond more ethical, but women are already aware that if you buy "fake" jewelry, you can get bigger stones, and generally that's just seen as trashy.

Real people do not behave as the homo-economicus of economics papers would have you believe. Especially when it comes to life-altering choices and the signs and symbols associated with them.

Short version: do not do this, for it will rebound on you horribly.

This is the intersection between Macro economic theories (op cost) and consumer behavior. And that too, one of the most emotionally charged decisions people make in their life times - marriage.

I would be extremely hesitant about using that particular gambit.