sorry, but i havent rng'd a snorlax at that location :/
I dont think that pokefulte will advance the frame, but it doesnt really matter
Use Elm calls to verify your frame before you encounter Snorlax. What I do to avoid NPC movements is I press B to exit the PokeGear after verifying my frame with Elm calls, which keeps the Start menu open which stops NPC movements, and the close the start menu and press A quickly in the hope a NPC doesnt move in that time.