mock_method.rb

# This is outside my control
class Foo
  def self.bar
    'Original'
  end
end
Foo.bar # 'Original'

Foo.singleton_class.send(:alias_method, :old_bar, :bar)
def Foo.bar
  'Mock'
end
Foo.bar # 'Mock'

Foo.singleton_class.send(:alias_method, :bar, :old_bar)

Foo.bar # 'Original'