May a party planner plan a non Jewish wedding and design decorations for the church ceremony?
Technically can plan the party from outside the church, but Jews may not walk inside it or even enter the church grounds. We are told to distance ourselves from avoda zara. In general, it is best to stay away from such areas, and IY”H Hashem will provide you with ample business from better places and events.
Rema Y:D 149-2, Shach ibid -1, Igros Moshe Y:D 3-129-(6), 1-68, O:CH 4-40 (26).