I can tell you from personal experience that it is highly unlikely that a powder brown tang will recover from ich on its own. They are extremely sensitive fish and very prone to ich, and it usually ends up killing them. Most fish are tough enough to fight off ich on their own, but PBTs are not in that category, sadly. They are also not suitable for a 90 gallon tank. They are very difficult fish to keep; one of the toughest tangs I ever tried to keep.
If you want to treat the fish for ich, like Hannah said, you will have to remove and treat all the fish in your tank. All your fish have been exposed and will now be carriers of the parasite. If you only remove and treat the PBT, it will get ich again as soon as you put it back in the tank with the other fish.