Есть такой код:

#include <iostream>

class A{

public:
    friend void fun(A a){std::cout << "Im here" << std::endl;}
    friend void fun2(){ std::cout << "Im here2" << std::endl; }
    friend void fun3();
};

void fun3(){
    std::cout << "Im here3" << std::endl;
}

int main() 
{  
    fun(A()); // works ok
    //fun2(); error: 'fun2' was not declared in this scope
    //A::fun2(); error: 'fun2' is not a member of 'A'
    fun3(); // works ok
} 

Как получить доступ к функции fun2 ()?