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I'd recommend doing a continuity test between battery ground cable and ground on frame. If it has continuity then you know it is negative ground. IF not, then it is positive ground. Let me know if you have questions, thanks.
I'm not able to supply any information that is going to help convince him but if you do the continuity test that should be enough. Let me know how it turns out or if you have more questions, thanks.
IF you still have old battery you may be able to see wear spots on it which will indicate how it was sitting in engine compartment.
No problem, let me know if you need anything else. Thanks again and have a good day.