How long have u had it since u got it from your friend and has it been eating well? Do you keep similar water parameters as the person u got it from and is the tank similar size to its last home? I have had a few mysterious deaths of new rays over the years with no verifiable cause. These are all things to consider in trying to figure out why its unhappy. I would definitely check your parameters and try to match them to what the ray was used to as quickly as possible, temperature as well. Unfortunately it seems to me from past experiences that occasionally some fish just overly stress from a move and can't recover. I hope this is not the case for u and give best wishes for sure. And hope fully as deadeye stated, nothing harmful was in the water during the change.
