The main problem - if he has hard water, then adding more buffers wont change that. Sure you can dump more crap into the water, and force the pH down, but the water is still going to be hard, and the Total Dissolved Solids is actually increased.
To make the water better for Discus, you need to take something OUT of the water, like most of the dissolved minerals. Then you can add a little bit of buffering minerals to the pure (and very soft water), just to give it some stability.
R/O filtered water is what is needed, then add a little "Discus buffer" or "R/O Right" to that.
Not getting into the tank size, and tank bred strains of discus can live OK with water that's a bit harder than ideal. But to thrive and breed they need soft and very clean water. Easiest way to keep water clean is a bigger tank and lots of water changes.
Ian