To reach Lake Acuity and help your friend, you first need to go north through Mount Coronet.
Get the Strength TM
Before you can explore Mount Coronet, you need to get TM096 Strength. If you don't have it, you should go outside and use the map to go to Solaceon Town, then go south to the Lost Tower, where you can get TM096 Strength.
In Mount Coronet, you can catch Chingling, Cleffa, Geodude, Machop, Meditite, and Zubat. You can use the Old Rod in the water to catch Magikarp, or use the Good Rod to catch Barboach or Magikarp. You can Surf to catch Golbat and Zubat.
Go west and check on the rock to find a Tiny Mushroom.
Go southeast to get an Escape Rope.
Go west from there, and look for the silvery rock on the left. Check on it to use Strength, then push the silvery rock to the left until you can go north. Push the northern rock to the north, then go west to get TM69 Rock Polish.
Then go back toward the east exit, but don't exit. Instead, go north from the east exit. Push the silvery boulder to the north, then go west and check on the brown rocks to break them with Rock Smash. Go west from there to get a Rare Candy.
Go east, then north down some stairs. Follow the path down to a lower area.
Mount Coronet B1F
Use the Pokétch to use Defog.
Go west and check on the brown rock to break it with Rock Smash, then get the Stardust to the left.
In this area, you can catch Chingling, Clefairy, Golbat, Graveler, Machoke, and Meditite. You can use the Old Rod in the water to catch Magikarp, or use the Good Rod to catch Barboach or Magikarp. You can Surf to catch Golbat and Zubat.
Go north from there, and go down the stairs. Surf northeast in the water to find a Light Clay.
Surf northeast to find a Max Elixir.
Go west from there and check the rocks to the north to find a hidden Ultra Ball.
Go east up the stairs and use Strength to push the rock out of the way so you can get the Revive.
Keep going east and follow the path, using Rock Smash on the rocks in the way, to get a Full Restore.
Go back the way you came, and go west until you can't go farther west. Go south and then east down the stairs, then north to get a Soft Sand.
Go back the way you came until you can go north. Keep going north, and check the northwest rocks to get a hidden Rare Candy.
Then go north, and up the stairs on the right.
Mount Coronet 1F, part 2
Go to the northeast, and check between the two rocks to get a hidden HP up.
Then go west to leave the cave and reach Route 216.