Are Moray Eels and Stingrays compatible in an aquarium tank?

My friend has an aquarium with sting rays in it and is looking to get some moray eels. The concern is that they will fight to the death or something when they get together, just wondering if they can coexist all cool like or not.

I have seen them co-exist before. The eel sticks to the rock, the ray to the sand.