It's a bit of a maze down there, but there are a number of trainers who she can ask for directions and/or battle against.

Directions, yes. She's on her way to Lavender Town.

She can get directions to the Lavender Town exit! It involves going up a couple ladders and down a passage and around a corner to where sunlight is streaming in through the cave mouth.

Yay, sunlight! Gwen pats Fern for being such a good flashlight. Good flashlight, best Pokemon.

Onwards to Lavender Town!

Lavender Town is adorably tiny and full of photogenic fog. The skyline is dominated by Pokémon Tower, the mausoleum where trainers lay their departed Pokémon to rest.

There's a missing person poster repeated in a bunch of places, saying someone named Mister Fuji hasn't been seen anywhere.

Oh dear. Well, she's here to see the Tower, and she's not really any kind of kid detective.

... Where was he last seen, though? Do the posters say that?

It's not clear where he disappeared from, but he spent most of his time at his home and at Pokémon Tower. The police are looking into the matter.

Yeah, the police are better at these things. Onward to pay respects to Pokemon.

The inside of Pokémon Tower is extremely spooky. Like, way spookier than could possibly be typical. There are definitely ghost Pokémon all over the place, and they're indistinct but clearly annoyed. All the trainers who hang out in here are a mix of irritable and hysterical from the bad vibes.