Once Ich is in the tank it may affect all fish, especially if water quality is allowed to deteriorate. So, as long as this will be a Fish Only tank treating all the fish with a copper medication would be a good thing.
However, water quality needs to be brought up to standards and kept there. That is done by weekly partial water changes. I would start with a few 10% - 20% daily pwc
's for a week to get water quality back to where it should be. That will dilute the copper and more will need to be added. That's one of the reasons you need a quality copper test kit to help maintain therapeutic levels in the tank.
You should have a serious conversation with the owner and let him know what he needs to do to maintain a tank in the store. If he's unwilling to do that, let him know that he just sell the tank and buy some pictures or artwork for the store.
Most of the tank maintenance companies I know of don't do a very good job. Maybe that's because they only come once a month instead of every other week. Maybe it's because proper maintenance is too expensive.
One place I worked at had a great looking tank for 1 week of the month. The company would come in and do a 50% water change, remove the dead fish, corals, etc. Add new fish and corals, etc. The lights were standard T8 bulbs so the corals started to die immediately. The company paid a monthly fee for this service and insisted the maintenance company knew what they were doing...