Donk bets are usually weak/vulnerable/drawing hands. If villain flopped a flush, we wouldn't expect him to take this line. I usually just punish these players, but this situation is somewhat different.
However, villain probably does have a flush draw, maybe with a pair, or some SD+FD combo hand. We could play this one of three ways....
1. We flat, and just let him keep betting. We call, unless a spade rolls off (my best guess of what V actually has, a FD and maybe a pair too. He may also have some combos of J-10o, A-10o, with one spade). If we call the flop, we will re-assess on the turn. Villain probably does have equity vs. us. There are other hands as well, I'll get to that...
2. We raise, get ready to play a big pot, and encounter some nasty variance. This option allows us to get full value for our hand IF villain has pair+FD or a combo draw. This option also allows for biggest losses. And our hand is really not that great here. There are many cards we don't want to see on the turn, or river. So if we raise, we have to have a good read.
3. Just call down, assuming no nasty turn/river cards peel. This option allows villain to bet all the way to river with inferior hands, and he will have no idea where he is in the hand.
I probably just call, and see what villain does on the turn. If he donks weak again, and the turn is unlikely to help him, maybe then we punish him.
However, we must remember, donk bets usually indicate weak/vulnerable hands. Villain could have some 2p hands in his range as well. Most people who donk bet are trying to set their own price, they don't want to be raised (but they usually don't fold easily). It is like a defensive bet. They are trying to slow you down. The point being, some of villain's donk betting hands may actually be beating us already (like K-Q). How to proceed is based on reads, and thank goodness we have position...
Donk betting really is a terrible strategy against strong players. I honestly don't know why so many people donk into their opponents. It is like a fish trend that's sweeping the nation (or planet). When a villain does this to me, 90%+ they are weak. Smart players will use donk bets against you once in a while.
And just for good measure, let's put ourselves in villain's shoes. What does he think we have? Does he think?
So yeah, just call, and see what V does on the turn. If the turn is a brick, we could consider just moving in or at least raising (if he bets weak again), and getting V to call with that FD or combo hand, when he has lost a ton of equity. However, on the flop we have about $1.85, and if we call, we have $1.60ish. Villains bet on turn would likely be around 30 cents, maybe a little more. So if we raise, we are pretty much committing ourselves, and then we have an awkward river bet size left. If we raise the turn, we could well just shove for max FE. It is possible we even fold some better hands, but not likely seeing how deep villain is.
I think out of all the things I discussed, call down mode is probably what I like most. Villains range really is pretty wide, and we can narrow it using position, and future bets on later streets. We keep the pot small, keep his air in, etc. But if we take this line, we must not pay off if the dude gets there. For example, if a spade rolls off, and the dude bets much harder, he probably got there. If a straight card gets there, and he bets hard, he probably got there. He will let you know if he improved, based on how he bets. You just have to be hip to the signs.
This is indeed a tricky spot, worthy of thought and discussion.
If anyone has anything more to add, I'd love to hear it. This HH made me really think.