he must have had an aquarium the size of a house ( you know the sea life aquariums, those giant ones! ) sturgeon get huge especially in the wild. would not even suggest kepping one in the pond unless it was about 6 ft deep and 12 meters by 20 meters. even that is kinda small.
There are a few species of Sturgeon that get no larger than a metre in length. They are fine in very large sheltered Koi ponds with lots of dissolved oxygen in the water, but no water weed because they cannot swim backwards (like sharks) and if caught up cannot untangle themselves. As for an aquarium your friend may well have kept a juvenile for a short time but I very much doubt that it would've lived very long.
My lfs has a sturgeon in an aquarium in his shop. It's about 3' long and is kept in a 10'X10' tank along with a couple of rays and an electric eel. Of course it's the centerpiece of the shop. It is impressive.
Appreciate your responses but according to various sites and shops there are sturgeons that will grow to a maximum 90cm - in a BIG tank surely this would be OK?