Carolyn Hax: Teen daughter wants space from parents but is upset when they give her space

That is, assuming you heard them correctly. Just to cover the possibility of misunderstanding, consider asking your fiance, his mom, or the opter-outer herself what she meant. Because what you overheard is pretty seriously weird, for one thing, which alone makes it worth verifying — and also because it helps to counteract anger with skepticism. You want your decisions to come from the best information available, and the best way to get that, typically, is to ask.