I just saved one sim from Grim after she collapsed from freezing. She was on the street in front of my sim's house and my sim convinced her to group up. It would take too long to walk back to my sim...
I've yet to see if it helped, but I gave all sims from my rotation cold resistant trait. Once I reach winter again I'll report the results. So far no deaths from the heat ( none of my sims ever died from overheating , but I gave them that trait just in case too).