You have to keep traveling in the game, just go dowon route 210 and earn hm03 cause she shows up later to battle with you. She will find you later in the game
mom
You can find her at the Contest Hall.
Beat the forth gym (fighting).
Go to Jubilife city. Then go through the left gate and surf to canalave city. Then beat the gym and continue your game.
canalave
in platinum she is in the Pokemon contest center
Beat maylene and wake first.
After you get the HM and earn the Fantina in the Hearthome City Gym
In Diamond, Badge 5 can be gotten by defeating Fantina in her Gym in Hearthome City.
Beat Fantina in Hearthome City.
In the Hospital. Because she has diarhea.
Hearthome City has the 5th Gym Leader. The 5th leader is Fantina and she uses Ghost-type Pokémon.