Should the fiancé of the father of the groom sit in the church with him or should he sit with his remarried ex-wife?

Where you sit will depend on the relationship he has with his ex wife . In most cases of divorced parents the Mother and her spouse/date/fiance sit in the front row and the Father/ spouse/date/fiance sit in the row right behind. The parents sit closeset to the aisle.

Unless there is a lot of turmoil . Some divorced couples can not get along at all. I would ask the groom what he thinks.

I agree with mydog and would just ad that regardless of where the father of the groom sits, you sit beside him.