Yes, there is a bit of hyperbole in the OPs post, but I interpreted that as a natural counter-reaction to the attitudes of the people in question. Also I thought it was plenty clear enough – 2 family members, female 28 and 40, went into the sierra, apparently without him so all the lectures on leading by example may not apply here, 10 miles from the road at 8000 feet with NO SHELTER of any kind with the intent of staying overnight. One down and one synthetic bag. They were inexperienced and based their choices only on a weather report that said there would be "no rain". That is fine all by itself since if we assume they carried everything else there is still a pretty easy route for them to get in trouble. Is it likely? NO. Would they probably make it out even if the worst happened? Yes. But it is still inadvisable for reason that, it seems clear, the people in question neither understood, nor were prepared for the actual condition that could easily arise. End of story.
I think it is ingenuous to ask if there was a large tree nearby, a cave and so on. Also I don't think whether they carried a pad matters, except to fine-tune how bad it could be. These are irrelevant details. It would be easy to assume that since they were relying on "no rain" they would also leave behind any real rain gear, and I think that could seriously effect things. Still I don't need that info for overall advice.
The fact is that the Sierra, in May, at 8000 feet, lows can easily be in the 20's. In spite of over-all weather we know that they might still get rained on for long enough to soak their bags through so that both the down and synthetic bag could become be next to useless, tree or no tree. Given what we DO know, then, it is *easy* to imagine the following scenario: early evening, dark, temps in the 20's, with soaked clothes and bags and possible high winds. Could they huddle against a tree and survive the night? Probably. Is there a route to them dying in this scenario, especially if one or more other things go south, as is more likely given their lack of experience? Absolutely. Do people actually die in these circumstances? It is not "common" or even justifiable, but unfortunately yes, all the time. Again, end of story.
But even leaving the issue of death aside, it would seem clear the folks that did this don't have even the scenario of spending a night in abject misery huddled under a tree below freezing with an unusable bag on the radar, and it should have been. End of story as far as I'm concerned.
My advice to the OP would be to run through this scenario for them without anger, and without being hyperbolic and/or patronizing. Tell then what *could* happen, and tell them what *has* happened to other people in similar conditions. Tell them if they are willing to risk a night of abject misery, then to take a space-blanket as an extra layer of insurance. If they want near certainty of reasonable comfort no matter what, then tell them to take a light tarp between them, and rain jackets. Loan them yours. Possibly just planting the idea that the worst-case scenario is worse than they imagine will make the difference. Then let them do what they want – I never thought forcing them not to go was even a possibility. I would advice you to tell them you think the minimum to insure reasonable comfort would be totally waterproof protection for their stored bag, waterproof ground sheet, small tarp or poncho, and a pad. The tarp and rain jacket will help in the case of cold winds regardless. It they are prepared to make it *though* a 30-60 min localized shower without having their bag and clothes soaked, and to sleep on damp ground, then they will be fine.
But by all means, you guys go on arguing about the the number of angels that can dance on the head of a pin.