I wonder if his apartment has gas heat and when opening the window to air it out he was causing the furnace to kick on and pump out loads more CO2? That would also explain the spikes during the cold overnight hours.
It depends on the model. High (>90%) efficiency, condensing furnaces are completely sealed off from the house: they take in air from the outside (via PVC pipe), combust, extract the heat, and exhaust the results (via PVC pipes).
* https://www.youtube.com/watch?v=lBVvnDfW2Xo
Older lower efficiency (<85-90%) furnaces suck in air from the house, exhaust into metal pipes (PVC would melt because not as much heat is extracted from the fumes), can be back-drafted and you have to worry about CO.
Basically: if the furnace has vents in which you can see a pilot light, it's the older style.