Modding Help How do I make an NPC have different dialog after their quest is fulfilled?

There's this avian NPC in the intro mission, standing next to the vending machine and craving an oculemonade.
I made a quest which allows you to give them that oculemonade. Simple enough, and it works just fine.
Of course, since those are the only lines of dialog the NPC has, he'll still be complaining about the lemonade after the quest is completed, which makes the entire thing pointless.
My question is, how do I make it so that he'll switch to another set of dialog lines (thanking the player for buying him a drink) after the quest is completed?