Sandrine can be found inside her home, which is located on Route 508.
Snadrine's Home

She is the leader of the first gym. She uses Ground type Pokémon with an average level of 18, rewarding you Ω500 and, like every gym, 10 Trainer Points. She can't be battled in her home, but is found on the gyms page in the Battle tab.

She gives you the two Quests, Higher Grounds #1 and Higher Grounds #2, each with a TP requirement of 10,000.

Completing those quests grant you the Rock Climb Ability.