I just thought I would mention this because its pretty cool... it's unlikely that it will apply to your friend but I'll mention it anyways...
If he happends to be growing in a basement near the hot water heater and it happens to be gas powered... all he will need to maintain CO2 enrichment is a controller. Unscrew the vent hood from the top of the heater and the hot exhaust will vent right into the basement(exactly like a CO2 burner), then plug the heater into a controller so it will come on only when the CO2 levels drop. 1200ppm from what I've been reading is the ideal, and 1500ppm is actually detrimental to the plants(causes extra production of ethelyne the same gas bananas produce), and more expensive.
That is of course only if your friend doesn't use too much hot water, or doesn't mind a cold shower now and then. It's an efficient alternative to the expensive burners. Make sure he seals the basement well first, and he'll need a dehumidifier.
Are you describing the excellofizz CO2 pucks? those pucks you put ina bucket of water and they bubble out CO2 ?
Honestly I wouldnt bother with those because CO2 needs to be controlled accurately to get the benefits.
Firstly you need to increase your temperature from normal limits to boost the plants metabolism.
If you don't do that while increasing CO2 your just wasting it.
The problem is then that unless you actually know you have your CO2 level within the proper enriched range than you can't raise the temperature without possibly cooking your plants. The CO2 allows the plants to partially close their stomata which enables them to handle higher temperature. Otherwise they will lose too much moisture too quickly.
If your friend isnt using proper regulated CO2 he can't count on it actually being significantly elevated, and he should be venting his exhaust to the outdoors as frequently as possible to maintain aptmospheric levels of CO2. Which means any added CO2 will be vented away and lost
So no, I don't think it is a worthwhile investment
BTW is CO2 levels are not properly controlled and exceed 2000ppm, it can severely reduce yields