I doubt that her magic can reattach the spinal cord.
Even if it can, I don't think it's as simple as "healing magic will work as long as the brain hasn't completely died". If her throat is slit, she starts quickly bleeding out, and her brain can no longer get oxygen since the arteries responsible for that have just been cut. Healing magic doesn't create new blood and restart organs. It just repairs flesh. So if Fran doesn't heal her within just a few seconds, she's a goner because her body will have already shut down.
Our modern techniques for restarting the heart and bringing someone back from almost complete death are taken for granted, I think.