I'm a bit lost on why you need to talk to Nga? She appears very briefly and I think there's no step in any quest that requires you to meet her.
You can keep the sleeping powder and the game doesn't force you to lace his water at all.
Sorry if i didn't make sense I wrote that very late in the night but what I mean is further throughout the bully's storyline. It says you can meet Nga and her grandmother at College to talk about how to get into his phone or something like than and I was just confused because when I bought the powder that option suddenly disappeared. But thank you for clarifying