3 handed - Mandatory straddle on the button

SB folds. Expert opens the BB. I defend 85off.

Flop gin 9 7 6. Expert leads, I raise, calls.

Turn 9. Expert checks, I bet, check/raise, 3 bet, call.

River 7. Expert donks.

12.5:1 for a call. I mean, you can't fold big pots HU right?

Nh

There are two reasons why he bet--he represented a nine on the turn and represented being afraid of a boat when he just called the 3-bet, and he's then representing an unafraid nine on the river, or he has a nine and is afraid you won't bet the river. There's probably a number of hands he could have on the turn that don't include a nine or seven (such as an overpair), but, if so, would he have then bet the river? Most players would check the river without a boat here. I don't know about your "expert". I suppose he conceivably have a hand that isn't a boat (and is trying to get you to fold a seven), but is 12.5:1 odds enough? Maybe not, but calling the river at these odds is at most a small mistake, where folding the best hand is a huge one. Better to make the crying call here, I think.

I don't think we're going to get any answers from the river play because we'll just get into an endless leveling loop. It's also possible that our opponent is going to bet 100% of her range here.

The operative question is whether we can beat any/enough hands that she plays like this on the flop/turn. And the obvious candidates are naked 8s and overpairs. Against an unknown but presumably competent player I think it is realistic to assume enough of those hands comprise her range to make a crying call.

No, that's not what an expert would do. This is an easy river donk from the expert because you should be checking back basically everything. You lose. Just call and make him prove it

the donk after the check raise is a billboard sign that says I would have re-raised the turn
I call and weep at the house and mourn for my counterfeited Yahtzee
folding would make the baby jesus cry that's a bigger issue