How long has your buddy had this ray?
If he just got it chances are its a fresh import. These guys do no ship well and are often kept in less than considerable conditions prior to purchase. Like mentioned earlier epsom can help, if the fish will accept black worms you can even place black worms in a cup of tank water with 1 table spoon of epsom salt and then feed. You can also add epsom to the water directly the same way you would aquarium salt.
Again, do not let your friend get to discouraged from this. These poor rays die on people all the time even when tank parameters are optimal. A motoro would be a better first option however housing a motoro down the line will be difficult if not impossible for most.
If your friend looses the ray and wants to try again have some Hikari prazi pro ready as well as black worms and or ghost shrimp. No body asked what the ray was being feed and I am making the assumption some sort of live food was being offered. With the black worms you can use the epsom salt trick I mentioned. With the shrimp I would recommend soaking flak food in a cup with tank water and epsom salt and then feeding that flake food to the shrimp. They will eat it and become a gut loaded food item that most rays will readily accept. The main issue with these guys when newly imported is that they are riddled with internal parasites and with the metabolism of a sting ray it can be almost impossible to bring them back and they simply waste away. Feed like crazy and keep your water clean, best of luck with the ray buddy