How would you handle this?

[PantsOnFire]

NL home cash game. Player A bets big on the river. Player B calls. Player A says clearly: "Nice call you got it.". Player B says "thanks" and fires his cards into the muck and scoops the pot.

Player A sheepishly flips his cards over and says he has 9 high and missed his flush draw. But he actually hit a weird inside straight draw which gave him the nuts.