1. Octopus, save the coldwater pacific giants, have a very short lifespan (under 12 months).
2. An octopus can escape through any hole through which its beak can fit...which means that your friend will want to seal the top of his aquarium so that no holes even as small as the diameter of a pencil lead, exist.
3. Avoid the temptation to get a blue-ringed octopus, which alsthough small and very pretty (and cheap and plentiful), are fatally neurotoxic and highly aggressive...these are a species only for professional aquarium institutions.
4. An octopus will take no time to hunt kill and eat every fish and invert in your friend's tank...even hermits and clams, so it must be kept very much alone.