[Nick C]

He may have a pair and a diamond. Or he could have a draw that just improved (with, say, J [img]/images/graemlins/club.gif[/img] T [img]/images/graemlins/diamond.gif[/img]). Possibly he has, say, 98 and planned to checkraise the turn but doesn't want to anymore since you might have a flush. Maybe his hand didn't improve at all and he just thinks that now you might fold. Or, of course, he could have decided to lead instead of checkraising with a turned flush.

Anyway, I wouldn't fold. Raising is an option, but if he's just got a draw and is going to fire again with it anyway on the river if he misses, then I prefer just calling. However, in this case, if you're ahead, there's a good chance he also has a pair, and he might pay off with that.

I would probably just call down, but there are potential advantages to raising. I doubt this guy's folding anything you'd want him to, though, so if you're raising for a free showdown, then I don't like it. I'd only raise against this opponent if I liked my chances of being ahead enough (versus a second-best hand my opponent wanted to show down) that I planned to bet again on the river on a blank. (If I thought he had a draw and would check-fold on the river if he missed, then that would be another reason to raise.)

If you had the ace of diamonds, then your equity versus Villain's range would of course go up, which increases the potential value of a raise. You can't fold to a 3-bet with the nut flush draw, though, so that option is out. So if there's a good chance he has a flush and will 3-bet it, then I wouldn't want to raise with A [img]/images/graemlins/diamond.gif[/img] K [img]/images/graemlins/heart.gif[/img].

Anyway, one thing to consider is that quite often a donk-bet indicates that the card that fell improved your opponent's hand in some way. A hand like K2o doesn't seem that likely (but who knows), so the fact that it's a third diamond is, I think, something we're really contending with here.