"She literally left a key for a total stranger to her home, without doing ANY due-diligence. Someone basically sent her an email, and she said "okey-dokey, here's the keys, have fun.""

I don't use Airbnb from either side, but isn't the big problem here that you can't do due dilligence? As I understand it (and again, as a non-user maybe I am wrong), Airbnb pretty much blocks you from knowing too much about your temporary renter as a measure to avoid you using them as a match up service and then doing the deal directly to avoid paying them their fee.