This is why I fled freshwater... lolThe thing is that Malawi Cichlids need a high pH. So if he is doing CO2 it's going to reduce the pH. That pH range (and water hardness) is also why many FW plants will struggle.
^And this is one big reason I jumped from Brackish to full Marine. Many more plant/algae options available.
It would be a balancing act then of CO2 for the carefully selected plants, and the right buffer and substrate to keep the pH up.