请帮忙解决c++error

#include<iostream>                       
#include<string>
using namespace std;

class Customer                                            
{
    friend void displayAsFriend(Customer);               
private:
    int custNum;
    double balanceDue;
public:
    Customer(int=0, double=0.0);
    void displayCustomer();
};
Customer :: Customer(int num,double balance)
{
    custNum=num;                                          
    balanceDue=balance;
}
void Customer :: displayCustomer()                  
{
    cout<<"In the member function"<<endl;
    cout<<"Customer #"<<custNum<<" has balance of RM"<<balanceDue<<endl;  
void displayAsFriend(Customer cust)                             
{
    cout<<" In the friend function: "<<endl;
    cout<<" Customer #"<<cust.custNum<<" has a balance of RM "<<cust.balanceDue<<endl;
}

int main()
{
    Customer onePatron(3815,259.25);
    onePatron.displayCustomer();
    displayAsFriend(onePatron);
    return 0;        
}