The thing about hypothermia is it can creep up on you over hours, even without rain - and it can easily happen in the 50's if you are especially badly prepared. Of course if you get hit with rain unprepared it can slam into you very fast. I mostly worry about people who just naively go into the woods for the first time. No matter how much you read about this, until you experience the onset of hypothermia, and the effect it can have on your reasoning, you may not take it seriously enough. If it is someone with enough experience to recognize the onset, as well as have a plan for whatever happens (including just walking out) and not just for the case they hope will happen, then I'm not worried.
So close to the trail head the main problem is a mental one. It's where you at first don't, and then can't, make the right rational decision on what to do that could get you killed in that case. It requires a bit of extrapolation that not all people seem to be good at. I think every year there are probably people who die of exposure from the sequence, "I feel ok so far, (later) I'm cold but not freezing... (later still) I'm really cold but I can handle it... (too late) I'm wafelye relly frezbum....(finally)gawp!". Should have taken action back at "I'm cold" of one sort of another. Same thing goes at the other end of the temperature spectrum. Every year there are 5-10 people who just walk down into the Grand Canyon in summer on a day hike who are mentally unprepared, make wrong decisions, and die of heat stroke and/or dehydration, or in the best case scenario get rescued by a passer-by.
On the other hand just buy them a couple of those Mylar sleeping bags and tell then to take them, just in case. Then in the worst case scenario they will be miserable, but warm enough. I love cowboy camping too - just need to have actual contingency plans beyond "the weather guy said it probably would not rain or get too cold". In the Sierra there is no such a thing as an "it is not going to rain" condition! All it would take is one localized 30-60 min afternoon/evening rain or sleet shower of the kind that happen every day in the summer at multiple places in the Sierra, regardless of the global weather pattern, to ruin that calculation.