Have you tried an isolation tank with high heat and metrodonazole? first off, the ray should have been in isolation/quarantine, if it is not, get it in a qt tank so that you can manipulate its environment easily. 40 gallon or so will be fine, bare bottom, good established biological filter, aeration, heat, some salt, no lights.
It can come back, just because it is skinny does not mean it is time to give up.
Run a course of Metro in an iso tank. Dose daily 1/4 tsp per 20 gallons, with daily 40 to 50% water change before each dose. offer food once a day and remove within 5 minutes if not eaten, easiest to feed right before water change. keep heat at 85/86F. feed live blackworms or candian night crawlers for best results. dose for 7 days.