I never take overnight gear on a day hike, but I do take enough clothing (iinsulation and rain gear) so I can stay warm and dry (if not 100% comfortable) if I'm caught out overnight. I do take all 15 of the "Ten" Essentials. Not all of these items are essential for every hike, but it's easier just to leave my daypack packed so I can grab it and go.

I agree, her big lack was not leaving an itinerary with somebody. I tend not to do that for day hikes, either, but I do carry a Personal Locator Beacon should I get injured and not be able to self-evacuate. While I bought it primarily to keep friends and family off my back (psychological weight saving), that feature alone made it well worth the price!