I dunno how far the girl can really be trusted. I mean, after the end of this author’s rain monster girl story, I can’t help but wonder if this is also a subversion on cohabiting/sharing a secret with a quirk monster (well, xeno) girl.
Like, it could turn out that the girl is lying about being able to return humans to life and is just saying that to see how far the guy will go to keep the secret of his “girlfriend”.
Only point I could see against this is that killing the gyaru seemed like a reflexive action, which would necessitate her planning everything out beforehand or her legitimately reacting like that, but also choosing to quickly incorporate it into her study on humans.