In C++ ,friend class allows us to inherit the proprieties of one class to another class(es) which is also possible using inheritance. And also in inheritance we can set different levels of privileges to the child class(es) depending on our requirements, which suggests inheritance is more useful than friend class. Then what more can friend class do which cannot be done using inheritance? Why is friend class introduced when there is already inheritance property which has even more functionalities? 
What is the difference between the friend class and inheritance consept in C++?
programming languages;object oriented;c
To declare a class (or a function) a friend allows the friend to access private members [the naming in C++ is not my fault!] (data or function) directly. They aren't inherited in any meaningful way. Yes, it creates a interdependency, but very different and a lot weaker than inheritance does.