It sounds to me like he should install a passive exhaust vent for now so as to increase the efficiency of his cooling system and stop using the CO2.
Then when he can afford a good minisplit to cool the room then he can switch the intake off or just set it to a higher temperature so it's a backup cooling system if the AC fails. If using a good environmental controller then it will disable the CO2 enrichment when venting at least. A controller with a day / night temp setting can allow him to set a lower night temperature and the blower would kick on when the lights go off to drop the room say 5 - 8F providing a daily exchange of fresh air to prevent ethylene gas buildup.
Know that when running a sealed room humidity spikes at lights out are a common problem if you have a large biomass. Sometimes the intake can help with that depending on ambient RH% and temp. A good dehumidifier is generally required during lights out unless you happen to have adequate ambient conditions and can use the intake to reduce humidity during the dark period. The AC pulls a lot of water, when it doesn't have to fight the lighting you need some means of controlling the RH%. So the dehu is pretty much required since you can't keep blowing cold winter air into a room with the lights out to keep the RH% down.
