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Yes and no, because the rule exists for more than just what you thought.
But it's more my opinion though indeed.
Prove that?
No, but if you claim something, which people doubt, you kinda have to prove it, otherwise everyone will just go in circles (like this very thread here).
How much you prove depends on how much you care whether people believe you.
As far as rules of the forum though, nothing brought up yet has shown that what he did was in any way wrong. That's the thing, what you think he should do is only as valid as what he thinks he should do as long a neither of you break the rules.
You think he should prove it, he feels he has proved it enough... neither can really force the hand of the other.