Since there’s literally nothing left for you to do in Reversal Mountain, agree and she’ll thank you for helping her, wishing you well on your journey.

On exiting the tunnel, take a look to the North in front of the large yellow bulldozer- a Shiny Stone is sitting there, giving you another evolutionary item for whenever you feel you need it.

When you climb the slope that leads into Undella, don’t head through the gate just yet- instead skitter off to the South and grab the Berry Juice waiting upslope from the entrance to Reversal Mountain.  With that in your bag, step into town.

The first building to the North includes a few people who say miscellaneous things, and the father of the Rich Family, who is looking for a rare item- unfortunately, he doesn’t specify what, and you haven’t got one yet anyways.

The next structure to the North is the Marine Tube, an underwater tunnel aquarium- leave that be for now, continuing East and down some stairs onto the sand.  A Pearl is hidden in the white beach sand here, and you can then follow the beach to the West, South of your previous path.  Step into the Pokemon Center first, though, and you can talk to a man about the sale price of Sitrus Berries.  This unlocks the ‘Shocking Shopping’ Funfest Mission.  Another man in the Pokemon Center will give you a Prism Scale for talking to him (a useful item if you have a Milotic, as it boosts the thing’s defenses).

With that done, return to the beach of Undella.  You may want to head through the city gate going North, but when you step in that direction, Hugh shows up out of nowhere, challenging you to a battle.

Battle: Trainer Hugh (4100 Poke)
Opponent: Unfezant Lv 39, (Fully Evolved Starter) Lv 41, (Fully Evolved Elemental Monkey With Type Advantage Over His Starter) Lv 39

Hugh cheers over your power once you beat him, declaring that the two of you will terrify Team Plasma into fleeing, and he’ll be sure to retrieve the Purrloin he’s been after this whole time.  He also encourages you to keep working on the Pokedex, then heads North out the gate.