Just go all the way to the top of the lighthouse. Talk to Jasmine and Ampharos (just to be sure). Exit the lighthouse and from there, head west until you see water south. Surf straight down then left. You should reach Cianwood City. Now go to a house with a sign next to it (I think it's near the gym or the Pokecenter) and that is probably the house where you can get the Secretpotion. If you get the potion, head back to the lighthouse and Jasmine will leave and go to the gym.