I don't agree with that, since it's a blatant government establishment of religion. However the county should be able to limit the people who offer prayers at the board meetings to representatives of religions that have a significant number of adherents in the county, or at least limit the frequency of their participation to something proportional to the % of the county population which belongs to that religion. Per the standard given by the county officials, a Wahabbi Muslim would be welcome to give the prayer, but not a Wiccan -- even though I strongly suspect (hope) that there are more Wiccans than Wahhabi Muslims in the Richmond area.
No, a blatant establishment of religion would be the First United States Church of the Christian God, with a tax levy on all citizens to fund the church practices and membership required to hold public office.
Local county officials deciding who to and who not to invite to offer a prayer is the prerogative of a free people and is only an establishment of religion in the most ridiculously open-ended reading of the principle imaginable.