Is your tank a FOWLR or a reef? They mostly hang out, so generally speaking space isn't their issue, it's water quality. How big is the eel now?
Mine is about 2.5' and I think it would be a bit cramped in a 92 if it gets bigger than it is now - I would say once it hits the 2.5'-3' range you will want to move it to your Dad's tank. They produce so much waste when they get to this size you are better off with more water volume.
I would encourage you to bury some PVC tunnels for it if you haven't already - just make sure you use 2" PVC - I used 1" PVC and he has never been able to enter them.