#include<iostream.h>
#include<string.h>

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=0;
    balanceDue=0.0;
}
void Customer :: displayCustomer()
{
    cout<<"In the member function"<<endl;
    cout<<"Customer #"<<custNum<<" has balance of RM"<<cust.custNum<<endl;
}
void Customer :: 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);
    Customer.display.Customer();
    displayAsFriend(onePatron);
}

#include<iostream.h>                                  //没有这个头文件，错！改为#include<iostream>
#include<string.h>

class Customer;                                            //类的声明没有符号，去掉；
{
    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=0;                                       //这里虽然没有错误，但是这个构造函数真心没用啊。。形参一点作用没有。。
    balanceDue=0.0;
}
void Customer :: displayCustomer()
{
    cout<<"In the member function"<<endl;                                     //cout没有命名空间，在前面#include之后，加上标准命名空间using namespace std; 
    cout<<"Customer #"<<custNum<<" has balance of RM"<<cust.custNum<<endl;    //这个cust是什么东西，突然跑出来的一个对象？不明白。
}
void Customer :: displayAsFriend(Customer cust)                           //该函数为类的友元函数，不是类的成员函数，不要加类的命名限制；
{
    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);           //声明类的对象时，怎么会有符号呢，去掉.
    Customer.display.Customer();               //这是什么东西啊。。是不是onePatron.displayCustomer();？
    displayAsFriend(onePatron);                //函数没有返回值。
}

真心乱，都是基本错误。

楼主错误太多了，基本的格式都没懂。
#include<iostream.h>                        //这里VC6.0是支持的，05以后的版本不支持，按楼上的说法
#include<string.h>

class Customer                                             //类的声明没有符号，去掉；
{
    friend void displayAsFriend(Customer);                 //看书上的定义
private:
    int custNum;
    double balanceDue;
public:
    Customer(int=0, double=0.0);
    void displayCustomer();
};
Customer :: Customer(int num,double balance)
{
    custNum=num;                                            //构造函数就是用来初始化的，不必要这里初始化
    balanceDue=balance;
}
void Customer :: displayCustomer()                  
{
    cout<<"In the member function"<<endl;
    cout<<"Customer #"<<custNum<<" has balance of RM"<<balanceDue<<endl;    //在类体定义里怎么会有对象呢？直接访问私有成员
}
void displayAsFriend(Customer cust)                             //该函数为类的友元函数，不是类的成员函数，不要加类的命名限制；调用的格式
{
    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;                                                                     //定义了返回值类型就要返回，不然有warning
}