There is no doubt to me who the donk is. I could see making a small bet in his place but not allin with the flush showing and him with just the 9. There were also cards over his 9s to pair. I think you slow played good after the flop. I like face cards suited myself but I can see why you didn't raise more preflop. With your chip stack the raise you made was a good percentage of your chips. Good hand and well played.
