An admin user can skip validations for password when modifying another account.

user.rb

class User < ActiveRecord::Base

  cattr_accessible :admin_account

  validates :password, unless: admin_account

end

users_controller.rb

class UsersController < ActiveController
  # If current_user is an admin make :admin_account the current user
  User.admin_account = current_user if current_user != @user && current_user.admin?
  #We create an attribute called admin_account then require password if admin_account is nil.
  #In the controller we set the admin_account to current_user if current_user is an admin and they are editing someone else's account.
end