Something very similar happened to me a couple of years ago. I returned to my slip after an evening sail to find a few college age guys working on an Oday 22 a couple of slips away. They had just bought the boat and wanted to talk about how little they had paid for it, and what a great time they were going to have over the summer. One of them asked me, "so we can make it to Newport from here in a day, right?" I was a bit surprised at the question, because Newport is a long, two day sail under the best of circumstances for a boat that small. I started to get even more concerned when they said they would motor if the wind died, and they had no idea how big their gas tank was (only three gallons, and they had no other containers). I spent the next hour showing them how to set up their rig and tune the shrouds and generally make their boat ready for sea; they had installed the boom vang as their mainsheet and had no idea what the mainsheet blocks were for, or how to set up and use the jiffy reefing. They had no charts, no radio, no food or water aboard. One of them had experience with bigger sailboats (or so he said), but they were all generally clueless and as sure of their immortality as only 20 year olds can be. They had to be gone from the dock by noon the next day because they didn't want to pay any fees to the marina. Anyway, by the time I left, I think I convinced them they needed to buy some water jugs, charts, and an extra gas tank before they left. I have no idea whatever happened to them.
The next day or so, I posted an email on another sailing website almost identical to yours: should I have done more? The consensus was that I had done all I could, and that I can't be responsible for others' foolish choices. Besides, someone pointed out, even if I were to call the Coast Guard, what would I tell them? "Keep on the lookout for a bunch of college students in a small, white sloop. I think they're incompentent sailors"? How helpful. I am curious to see what others think of your predicament.