To my knowledge, Juma ferrules are no different than other ferrules, as far as polishing them goes. I use cue wax and/or bee's wax to polish plastic ferrules and burnish the sides of tips, and it's always worked great for me. Perhaps there are other better polishing products out there.
Is your friend a "dirty" chalker? Meaning, does he hold the cube of chalk in his fist and grind it into the tip thereby capturing a pile of chalk dust in his hand which probably gets ground into the shaft and ferrule during his stroke? Or, as he chalks does he swirl the chalk around tip and rub a ring of chalk into the ferrule? If so, that'll kill *any* ferrule.
Also, how often does he have his shaft cleaned? Once whatever product that was used to polish the ferrule and/or seal the shaft is worn away, the open surfaces are susceptible to chalk dust, skin oils, etc., that will lead to a dirty shaft and ferrule. Periodic cleaning not only cleans away what is currently on the shaft and ferrule, but it helps to keep new stuff off (for a while).
Anyway, I don't think you did anything "wrong" polishing the ferrule. Your friend probably may just need to clean his cue more often.