If your friends use space heaters or baseboard heaters, ensure that they're kept at least a foot (30cm) away from anything combustible.
It used to be commonplace to use heaters to dry or warm clothing. This is actually a common cause for house fires; you should never put clothing over or directly next to a heater to dry.
Never let your friends use an oven or clothes dryer to heat their home. If you see a friend doing this, you might want to consider getting them a space heater as a gift.
Heating units need to be properly maintained. Let your friends know if you spot any cracked, frayed, or loose wires; these are easy to miss if you use the unit often, and might stand out more to visitors.
If there are any fuels kept on-hand for kerosene or oil-fuelled units, they should be stored in an approved container or tank. If you see a friend using an inappropriate container, say something! Improper containers can leak liquids or gasses that are dangerously flammable.
Anything that burns fuel must be ventilated properly. Any installed ventilation, such as chimneys or vent ducts, must be inspected regularly, and any portable units should have a built-in CO shutoff (which is common in newer models) or be used in a well-ventilated area. If your friends have heating units with installed ventilation, you should remind them to have it inspected and/or cleaned every fall.
