Martha
There was something about Acerola's voice and general excitement that made Nanu accept her request. He let her invite him to visit the local haunted house together. It wasn't that he was scared, he'd never be afraid of ghosts with Acerola around. Maybe it was him getting so close with his Dark-type Pokemon that made him just a bit more careful about places like that... He's heard stories about ghosts liking dark places, even if the Dark-type moves were effective on Ghost-types...

No matter the underlying reason, he went to the house to wait for his companion there. Nanu promised her to meet her there so they could get inside together. He hoped Acerola would actually enjoyed her time there.
Paula
*Acerola, having gotten there hours ago, was already strategizing on how to scare her friend in the best way. Peeking out of a bush, she spies on Nanu, wanting to sneak up on him. She rustles the leaves around to try and alert him to walk to over. She giggles to herself a bit in the bush and waits for her plan to unfurl*
Martha
Oh, if only Nanu had any of his Pokemon out now... But he didn't have any of his companions out, not even the Persian. She didn't really want to do much that day, so Nanu left her in the ball and just went there on his own.

And of course he fell for the trick.

Curiosity killed the Meowth, or so they say. Nanu was just like one of them, so he went over to the bush soon after he heard the rustling of the leaves there. It honestly reminded him of the way he's found one of the Meowths before, so there was no way he would expect Acerola instead of a cat Pokemon hiding there.