"Pokemon Platinum: Haunter not evolving"

I recently started playing old pokemon games on the emulator, and always wanted a Gengar which was never possible. Today, while playing pokemon platinum, I reached this lady in Snowpoint city offering a Haunter for a Medicham. So I went to Mt Coronet to find the highest level one. When I traded, she traded it with an everstone, because of which, it did not evolve. However, I took away the everstone, and used a rare candy, but it still did not evolve. What do I do? via /r/pokemon https://ift.tt/3Gg4DhR