Stub method with instance variable

Lets say I have a controller: UserController

and there is a method:

before_action :add_friend

def add_friend
   @friend = Friend.last
end

How can I stub add_friend and create instead another instance_variable called @friend2:

@friend2 = Friend.first

So that:

@friend == nil

and

@friend2.nil? == false

What I tried was:

 @friend2 = Friend.first
 UserController.any_instance.stubs(:add_friend).returns(@friend2)

But that didnt worked! Thanks