It all happens so fast. Rokuro sees/feels Link move towards the cup, and before he can reach out to stop him the goblin man has thrown back the entire contents in a single gulp.

Witnessing Link's reaction, which was similar (but far less self-preserving) than his own, all he can do is roll his singular eye with a sigh, still dabbing daintily at the corners of his own mouth with a napkin.

"If you do die of poisoning, you will have come by your fate honestly." Rokuro wouldn't actually let Link die, he does have the power to stop the bloodflow in Link's body and limit the spread, but a little discomfort? Rokuro isn't about to call upon the power of a god for such a pittance. Perhaps the blonde would learn his lesson and stop sticking mysterious objects/liquids/animals directly into his maw.

"How do you feel? Are your ancestors beckoning you to their sides, or are they simply calling you a little fool?"