class ApplicationController < ActionController::Base

  def is_coach?
    user_signed_in? ? current_user.is_coach : 'false'
  end

  protected

end

undefined method `is_coach?'

#config/navigation.rb
SimpleNavigation::Configuration.run do |navigation|
  navigation.selected_class = 'current'
  navigation.items do |primary|
    primary.item :home, 'Home', '/'
    primary.item :home, 'Clients', '/clients', class: 'special', if: -> { is_coach? }
  end
end

<%= render_navigation %>

def is_coach?
   user_signed_in? ? current_user.is_coach : 'false'
 end