[IHE]

I kind of like your line with a bluff since his hand looks like an ace or medium ten. And it would be a really sick value checkraise with a hand like T9 or AK if he is a decent hand reader for the same reason. Maybe it is too advanced for an unknown 25/50 HU donkey, but it is conceivable that he can bluff 3-bet this river expecting you to either bluff or checkraise very thin in this spot. Just something else to consider, although maybe not relevant since you suspect he's mediocre.