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.