Hi Sago,
Maybe your friend who was worried about leaving her seventeen year old alone in house for a few hours was really worrying her own, or lack of, parenting skills.
It reminds me of one of my then teenage daughter’s capers. She would have been about sixteen or seventeen when she and her best friend had a party while I was away. Probably partying myself. They didn’t actually tell me about this party until some years later. The two of them systematically took pics of every room in the house and then locked the valuables and breakables in the room we used as a junk room. Apparently they had a whale of a time and I believe the police only turned up once. Daughter had warned neighbours that they were going to party and that they were welcome to pop in for a pint or two of snakebite!! On my return I thought they looked a bit second hand but nothing was mentioned and life carried on. After some years had passed this story was retold while we were all reminiscing about what we got up to as teenagers. My headstrong goth teenager is now a very well qualified and much respected art psychotherapist and her friend is a literary agent. To this day I haven’t told them that I knew what they had got up to. How did I know? Well the house hadn’t been that clean and tidy since we moved in shortly after my husband’s sudden death a few years earlier. Everything was per the pics in their phones, most of the windows were open to get rid of the stale smell as good as any proper old fashioned public bar! The absolute final give away was the washing inexpertly blowing in the wind in our small walled garden. If I ever do let them know that I knew, I would compliment them on their forward thinking and respect for our home. That day I recognised that I had got SOMETHING right and by not saying anything I let them think they had got a “win” , as they say in America