Yes. But it depends a bit on the climate where you live. What you're looking for is an air-to-air heat pump. Basically it replaces the outside unit of your traditional air conditioner. But it can heat or cool.
If they only have a gas furnace, then they may not have an outside unit currently. Piping a refrigerant line and signalling from where the furnace is to a suitable location for the outside unit may be easy or hard depending on their home.
Of course, if they have a furnace and an air conditioner, it's a lot easier to change to a heat pump and that's it (or more likely, a heat pump and just in case resistive heating)
Of course, if they have a furnace and an air conditioner, it's a lot easier to change to a heat pump and that's it (or more likely, a heat pump and just in case resistive heating)