It may not live long. Try to give it the best life possible for however long that may be. Do you or they know what happened to it? Why would they send you or anyone, one like that without warning first?
Did you just get them today? Is the other one eating? They or he may just need some time to get accustomed. See what others think. But if he doesn't eat in a few days, I think I would think about putting him down then letting him suffer. I am not sure about turtles. But the most humane way to kill a fish that is sick or dying is to put it in the freezer. I wouldn't let it suffer a slow terrible death. I will start a thread asking the question. Maybe it will give some other ideas. Good luck, sorry that happened.