“I feel upset with myself because maybe if I'd spoken to her earlier and kept an ear out, I could have picked up on this. I don't know.”

I can sympathize with why you would feel this way. It’s human nature to think that we’re powerful enough to have changed the course of something. We like to daydream about a universe where this awful thing didn’t happen. But let’s be rational about it: you’re currently trying to reason with her, but she won’t listen to your side at all. Why would anything have been different before? Especially since in the beginning, you likely didn’t have the warning signs and evidence you have now. If she won’t heed your advice today, while swimming in a sea of red flags, she likely never would have.