Yes, copper is a proven treatment for Cryptocaryon irritans. However, there are several drawbacks (one advantage) to using copper. It is stressful to the fish, suppresses immune function, toxic and some species do not often survive prolonged exposure. The advantage is that copper is effective against amyloodinium. IMO, Cupramine is far superior to any other copper-based medication. I have used it successfully to treat coper-sensitive species such as puffers, lionfish, mandarins and dwarf angels. I no longer use copper as a first choice since I believe hyposalinity has several advantages. BTW, hippo tangs seem to be about the toughest fish to cure of ich. I suspect that sometimes the white spots that are seen on them are not actually ich.
The last time I checked there were only around 100 veternarians qualified in aquatic medicine nationally. Your friend may be one of the few, but most of them know little about treating fish disease. Of course some of vets do own aquariums.
Terry B