What is a cheap way to measure co2 or o2 level in air?
Well, MQ 135 sensor is very cheap general air quality measurement device. It costs around 3$. There is another similar on AliExpress, not even worth mentioning. What is your budget? Gravity from Adafruit seems proper, yet it is at 56$. One that seems actually serious is K30 sensor, but that is around 100$. That 3$ stuff is general, and will work when temperature is constant, humidity is constant and Mars is aligned with Venus. 

I think 56$ is ok if they want to have something proper. Remember that you get what you pay for.