The fact that you “feel like a total failure for not being able to show her the folly of those decisions” shows that you’re taking this coming Real Estate crash a little too personality.

You’ve told your neighbor already. Just chill and let her be. She’s gonna do what she’s gonna do. Me, I would only tell my neighbor if she were my friend also. I’ve learned my lesson; never deliver bad news to strangers. They’ll resent you.