Keep going East and you can run into a martial artist.

Triple Battle: Battle Girl Chan (1184 Poke)
Opponent: Riolu Lv 37, Gurdurr Lv 37, Riou Lv 37

Divert to the North again after facing Chan and you can snag a Revive from the dead end here.  Return quickly to the path along the South wall and resume your Eastward direction to collect a valuable Star Piece from the stalagmite at the end of the tunnel.

Now it’s time to return to the passage South of Kiyo and Marcus that you ignored earlier.

Head through the hole in the wall and you’ll find yourself in a tunnel that runs South and East, with an exit at either end.  Go through the South door to pick up an Escape Rope and a Smoke Ball.  The Smoke Ball is an odd hold item that ensures the holding pokemon can always flee.  This includes not only absolute Run success from wild battles, but also ensures you can switch pokemon in trainer battles.

The other exit doesn’t actually go anywhere, just leaving you on the empty overlook by the Strange House.  Since you already explored the building, ignore this dead end and go back to where Bianca said to look around and you found a Doctor.

Head down the stairs here at last, and you’ll find a low chamber with a pit in the center.  Bianca moves forwards to check it out, wondering aloud if this is where Heatran, a legendary pokemon, made its lair and effectively started the creation of Reversal Mountain.  She tells you that Heatran is a pokemon with blood like magma that runs through it... and then is done here.

Climb back up the stairs, then head West and loop North, following the outside edge of this floor of Reversal Mountain.  You’ll presently reach the North wall again, at which point you should head East as far as you can, at last passing through the doorway that grants you access to the far side exit of the mountain.

On reaching the doorway, Bianca will stop and point out that the exit here leads to Undella Town, and she wants to keep looking around a bit more.  She asks you if you’re ready to split up for the time being.