Great story. I know exactly what they went through and it happens all the time on Mt. Tam.
When I lived in Californian, I was involved with the Marin County SAR team. We would be called out at least every other month for a lost hiker or mountain biker on Mt. Tam. Hikers would start out exactly like our two friends did in the story, it might be 75 degrees and sunny out at the base of the mountain, then by sunset the fog rolls in and you cant see your hand in front of your face. Many did not have extra clothes, map, flashlight or even water.
In fact my first search was for an elderly man that wandered off to the bathroom from the parking lot. We never found him, he died a few feet from the trail and some kids found him 8 months later. I still remember his name and that almost 20 years ago.
Mt Tam is beautiful, but you need to be prepared and know where you are going and how to get it. It is very deceiving since it seems like your are so close to homes and parks.
Always be ready.
Ready
_________________________
"Do what you can, with what you have, where you are" Theodore Roosevelt