I am so thoroughly confused by the syntax you are using. Did you by any chance mean something more like this?

class Hero
{
	// Awesome and clean code
};

class Spell
{
public:
	// Awesome and clean interface
private:
	friend class Hero; // Ouch, not too awesome

	Hero &hero;
};

class Heal : public Spell
{
public:
	Heal(const Hero&);
	
protected:
	// Helper functions
};

Well, in order to pass a modifiable Hero pointer to Heal, you need to watch out for a couple of things.

One, you are passing a const Hero, which means you won't be able to modify it. Remove the const keyword if you want to modify the object.

Two, as far as I can tell, you're passing Hero to the wrong object. The member variable hero in Spell is declared private and trying to assign a value to it from Heal will generate a compiler error.

Try fixing those errors and see if you can get it to do what you want.

What error is it giving you?

I am concerned about the ownership issues that this constructor is causing. How are you going to ensure the Hero reference remains valid? Can you guarantee that the Hero will not be deleted (presuming it is dynamically allocated), leaving an invalid reference? Servant's alternative approach is probably what you should be doing.

You should probably be using smart pointers or values, as it is clear you don't yet understand how raw pointers behave. You'll still need to learn the concepts, but smart pointers at least will help you start with good practises and, used exclusively, hopefully limit the kinds of mistakes that are possible.