Sadly, carbon monoxide poisoning is an insidious threat which has killed many pilots in the past.  Just as one should always have carbon monoxide detectors anywhere there is a combustion heater, I always recommend that anyone flying a small plane carry and use one of the various types of small, portable electronic detector/alarms which are readily available.  Unlike the cheap, slow responding and hard-to-see chemical disks folks often buy and stick on the dashboard as an afterthought, the electronic monitors have quick response times, loud alarms, and flashing lights.  I carry a small "Pocket CO" on my keychain.  To me, the $140 or so that I paid for it is well worth the investment in the safety of myself and my passengers, and the peace of mind that it brings while flying is priceless.

Deepest condolences to the pilot and family.  If anything could be gained from this, at least let it be a greater awareness of this threat which might save the lives of others in the future.