I'm normally on the freshwater side of the forum, but noticed this thread.
I must say that the idea that no sentient creature should be kept in a cage is quite untrue when it comes to conservation of many species. Most great apes are in zoos, as well as elephants and many other animals capable of recognizing themselves in mirrors, as well as complex emotions. Without the efforts of these zoos, many of these species would be much more in danger in the wild than they currently are, and many are already at great risk in the wild.
However, in this case, the animal is not going to be contributing back to its species through captive breeding, so it's more for entertainment. Its not really my place to say whats right and what's wrong and only that power is in the hands of the owner. If they really wanna try it, go ahead. But please think carefully and maybe consider something that won't almost surely end up out of the tank
Octopi are truly wondrous creatures and their intelligence is something to behold. They rival a small child easily in intelligence, creativity, awareness, etc. and are easily one of, if not the smartest invertebrates on the planet