Cloudy water could be an indication of higher ammonia levels or a bacterial bloom, a bacterial bloom isn't harmful to fish though so that wouldn't be a problem. If it was ammonia the other fish would be reacting to it as well so I'd lean towards a disease of some kind and have them remove the fish. It sucks for sure but someone mentioned at least putting the fish in a bucket with a heater and air stone to give the fish a chance. I can't speak on Angel fish but I know I've brought my clownfish back from what I thought was certain death on two separate occasions. Clownfish are pretty resilient though so probably not a good comparison. I always keep a bag of chemipure on hand just for incidents like this one where it could possibly help improve water quality enough to make a difference. Good luck man, hopefully he pulls through.