Not sure it would count as saving someone's life, but during one of the last "polar vortexes" that made their way down VA this past winter and where below 0 temps were possible, around early January, i was out backpacking on the AT.
I expected to be the only person, or one of the only people out there, especially didn't expect any women as i know a lot of women tend to be significantly colder sleepers and generally less tolerant of cold.
When i came upon a shelter, i was very surprised to see a lone woman there. More over, i was surprised to see what she had for sleeping gear–a fairly thin, synthetic sleeping bag, and a cheap, not too thick foam pad.
I considered her way under prepared to be out there in those temps (i don't think she did though). Granted, if she had gotten overly cold, she could have hiked through the night to stay warm, but having no car, etc, she would have had to sleep at some point, and it was still fairly cold during the day too, she had been out there for a few weeks or so and was fairly tired.
She asked me towards the beginning of meeting her if i would sleep in her tent with her (i had a tarp) for warmth. I did. I shared the 0 degree, wide enlightened equipment quilt i brought, let her use my NeoAir all season pad, and we ended up cuddling. Like at home with my wife, i got the short end of the stick with the blanket (quilt) and often didn't have much covering me. The general situation was a bit awkward at first in some ways, but thankfully, i'm in an open marriage so my wife is fairly secure with herself and in the relationship to begin with. (Neither of us is promiscuous by any definition and i'm especially pretty inactive/passive in that area with other women to begin with).
Anyways, can't say i saved her life for certain or technically, as i think there were some options, but if she had been stubborn while alone, she could have gotten hypothermic easily, and i can say that i definitely made her night much more comfortable than it would have been if i hadn't been there. She was still often cold with all that extra warmth/insulation, so i can only imagine how miserable she would have been by herself with just her gear.