A visible object is an object that is visible for both players, if only one player can get informations, it can't be free information.

I can kind of understand the counter-argument - while I'm searching my opponent's library, only I have access to that information, not them; the library isn't being revealed. And if only one player has access to the information, then it can't be free or derived, which means it must be hidden. So we can't obligate the opponent to help with any part of it.

That said:
(1) Players are always allowed to reveal hidden information that they have access to. If I search my opponent's library, take a card out, put it on the table face-up, and say “what is that card?”, then I've not committed any sort of infraction here (unless you want to be extraordinarily pedantic and get me for rearranging the library, which one can technically only do during a shuffle, not a search).
(2) Words like “visible” that aren't defined game terms, the way “object” is, take their plain English meaning. So an object (which this clearly is, since it's a card) that both players can see the front face of, seems to be “visible” to both of those players. Which means it's a “visible object”.
(2) If AP did this, and NAP refused to help identify the card, then I'd ask NAP both to identify it, and what they intend to gain by not helping AP here. It seems the only advantage would be to hope that AP is satisfied with not knowing what the card is instead of calling for a judge.

I don't believe that I would issue NAP a penalty here, certainly not the first time that this happens. MTR 4.1 does allow them to say that they're unable or unwilling to help with Free Information. But if that's the case then I'd explain that NAP needs to be the one calling a judge, to explain why they're unwilling to help identify the card. If a player did this more than once at my event, without calling the judge themselves, I'd pull them aside for a conversation that would likely end in either a USC:Minor for not following the earlier instructions or a CPV for refusing to help with Free Information (and not calling a judge themselves to explain why).

Since I can't edit, and there are a few different angles of discussion and questions, could we get an on this please?

Isaac’s answer is marked as Accepted. That IS your ‘O’.

d:^D