Did you get it straight from the supplier or did the shop throw it in one of their tanks first? I would have separated her from all other fish and kept in complete darkness for several days.
I would never buy a ray before watching it eat. Ever. Especially a transhipped ray. Now it has gone halfway around the world and through 2 very stressful acclimations rather quickly. Prob won't eat for awhile, and your saying it's already skinny. Not a good sign. Hope for the best.
Whenever you are getting fish from non trusted sources, ie other contries, you should never introduce them into your main system with other thriving fish. Quarantine it alone if possible and in a separate system.
+1
You got a tough road ahead. This ray should be by itself for a period of time to have any chance of survival. Good luck.
small update this morning. found her moving about in the tank. slow but moving. are doing fine. i just poured in some prazipro inside the tank just for precaution this morning. totally slipped my mind for quarantine. i dont quarantine every fish (which is a sin). She seemed to have either shed or the flesh just fell off her disc where the rot was. seems to be healing a little. her ribs are showing but nose looks a little better than yesterday. Going to try and feed and see how she does. All of you guys are absolutely right, i shouldnt have done it but she is a beauty... and if she pulls through it will be all worth the risk xD water currently at 79 degrees, did large water change and replaced filter floss. added melafix and some salt yesterday.