A carbon monoxide detector or smoke alarm is a thoughtful house warming gift. Locate the carbon monoxide detector near bedrooms or where people spend time and where it can be heard if it goes off. If your alarm sounds leave the house and call 911 from the neighbors.
Furnaces are not built like they were in the old days. Metal heat exchangers are made thinner each year and what used to be a 20-year appliance is now only safe for half that time.
I have read some horror stories about problems that people have endured before they discovered CO was the source of their poor health. Some indicators of CO poisoning imitate the flu. Other indicators are headaches, nausea, drowsiness and fatigue. High levels of exposure to CO can cause permanent physical damage and even death.
If you are worried about their safety, get them a gift certificate from a professional heating contractor for a furnace checkup with a test for carbon monoxide. To add to the gift, get them a carbon monoxide detector for the house. They can be purchased for as little as $25 and up.