[TheWorstPlayer]

OK, maybe not correct. But at least quite close to correct. He also had a gutshot straight draw, right? (I'm too lazy to check the hand.) But I think Grue's point is that even though this guy is a donkey, he seems to be quite passive and your evidence of his donkishness is all from him calling with marginal hands, not overbetting hands or bluffing too frequently. So although if you check you may induce a bluff from complete garbage, it doesn't seem like he has complete garbage here. It seems like he has a flush or some worse made hand. So he is going to check the made hands and bet the flush. So you probably can check/fold. But I think that is leaving a lot of money on the table against this super loose opponent, which is why I think you should bet the river. But this villain simply does not seem too likely to bluff raise a marginal made hand on the river here. So if you bet $600 and he raises, you can be certain enough to fold knowing that he hit his flush. That way you make the most when ahead and lose the least when behind, IMO. Against a more normal opponent, it's a bad line as fsuplayer and others pointed out, but this guy seems so bad that it is the line which most takes advantage of his weaknesses (looseness and passivity).