Как вариант:
Класс вторичной формы.
#include <QPushButton>
class MyForm:
	public QWidget
{
	Q_OBJECT
public:
	MyForm(QObject * parent = 0);
public slots:
	void HideButton();
private:
	QPushButton * button;
};
MyForm::MyForm(QObject * parent):
	QWidget(parent)
{
	QPushButton * button = new QPushButton(this);
}
void MyForm::HideButton()
{
	button->setVisible(false);
}
Класс главного окна.
#include <QMainWindow>
class MainWindow:
	public QMainWindow
{
	Q_OBJECT
public:
	MainWindow();
};
MainWindow::MainWindow()
{
	QPushButton * button = new QPushButton(this);
	MyForm * myForm = new MyForm(this);
	connect(button, SIGNAL(clicked()), myForm, SLOT(HideButton()));
}