This is obviously a very unusual spot, which does not come up very often. So learning how to play it perfect is not the most important thing in improving our game. But looking at this from an MDF (Mininum Defence Frequency) perspective, then the opponent is betting 4 X the pot in a situation, where both players are at least playing the board. This means, that the unexploitable strategy is to fold something like 7/8 times, because that makes his bluffs break-even. This is assuming, we never have a flush, but thats probably fairly reasonable, since I dont see, why we would be checking a flush on all three streets.
So the question becomes, which hands other than flushes are the best ones to call with? And I think, its quite obvious, that we want to call with hands, that have a spade in them. But that is almost half of all our hands (a bit less given there are 3 on the board). So we cant call with all hands, that have a spade in them. And I think, its better to call with those, that block the flushes, which are most likely to be slowplayed. Which is the nut flush and the straight flush.
So from the GTO perspective the hands to call with are probably those with A, K or 9 of spades in that order. Are since we are supposed to be folding so much, that probably enough? Its definitely a mistake to want to call here, because "LOL I am playing the board, and he must be bluffing", which it seems like, a lot of people want to do.
And then there is the exploitable perspective. Where yes its kind of weird, he played a flopped flush like this. But slowplay is not that uncommon in the micros. What is uncommon though is for people to be overbluffing in spots, where the balanced line mean, they are already bluffing almost 90% of the time. Its a huge risk for Hero to call with very little reward.
So even without doing all this analysis Hero can just fold, and if he got bluffed, then good game to the opponent. It does not really matter, especially since Hero did not even voluntarely put chips in the pot until the river. 53o would have been an easy fold, if Villain had raised preflop. And as a rule of thump for free plays, if our hand is so bad, we would have folded preflop, then we should not look to put in chips postflop either, unless we hit the board really hard.
