So you did not realize this when your are trying to house a probable 24inch+ african freshwater shellfish carnivore with specialized hunting habitats which is typically a primary specimen fish for a 500gallon+ tank ? I suggest you return it, I certainly do not see a dilemma but a lack of appropriate research. Is it an African arrowana its housed with ? Heterotis niloticus ? I doubt it.
You know what? There are not that many of us that successfully keep large Mbu (I assume you do?...) but the question I always ask myself whenever I see posts like yours, the whole "You should know better" or "Didn't you do your research ahead of time?"-thing, is "How does this benefit the fish?".
The TS apparently already has the fish and nothing you or I do is likely to change that, so rather than telling the guy he made a huge mistake (and haven't we all at some point or another?), why not use some of your experience to guide him in the right direction rather than just blasting the guy? Do you think is puffer is going to be better off by you jumping down his throat? Do you think the puffer is going to get better care if he returns it to some crappy local fish place? I know that no Mbu I have ever seen in a fish store looked as good as mine.
Pointing out everything a person does wrong is easy. Anyone can do that because it requires no effort on their part. Personally, I would rather show someone how they can improve their situation but maybe that is just me.