It could have been stung, but it's so unlikely. Many, many people keep all sorts of stinging corals with fish. The fish just know not to go near them. Also, if indeed it was stung by the coral, It might have a mark on it or something, but not be dead. If the nitrates aren't high enough to effect the corals, it's not going to hurt the fish. Keep looking. Stray voltage perhaps? Ever get a shock with her hand in the water?