There's no telling what it could be. I'd say you've got around a 50/50 shot st best. I've had them come back from that, but lost them as well. In my experience, the trick to Haddoni is a VERY seriously slow drip acclimating, 12hrs or more. They just won't tolerate any rapid changes in their water chemistry and often die a slow death after that initial shock. This is why they don't generally ship too well.
Wish I was more helpful/optimistic , but a sick/stressed carpet... there just isn't anything to do but wait and see. I strongly advise against trying to medicate or move to quarantine. It would only shock it further.