None.
A full grown green spotted puffer will require a 30 gallon tank BARE MINIMUM. I'd suggest a 55 gallon. They are also brackish as juveniles and require full marine conditions as adults.
I'd tell your friend to look at some different fish.
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Current setups: 1800 gallon koi pond, 10 gallon planted, 150 gallon reef, other FOWLR tanks
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