Correct. The only way CO2 can kill you is it becomes concentrated so you do not get enough oxygen. Good catch. Five stars for you.
So, though he needs to go through the troubleshooting ideas you guys have given him, I tend to think it will be something coming in the air vents.
In 1998, we moved to McAllen from the Midwest. We rented a big yellow truck, loaded it up, and put our big Bonneville wagon on a trailer behind. When we stopped at night, I heard a noise, and when I investigated, I found our CO alarm in the back of the car, and enough CO had gone into the car, on a trailer behind the truck, that it fired off the alarm. So, I tend to think the fact the CO alarm did not ring is because it isn't CO, but fumes as someone has suggested.
But, better safe than sorry. Give yourself plenty of air in that car until you find the problem.