public bool form2Cerrado = false;

private void Form2_FormClosed(object sender, FormClosedEventArgs e)
{
     form2Cerrado = true;
}

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
 
namespace Ventanas
{
    public partial class Form1 : Form
    {
        Form2 f2 = new Form2();
        Form3 f3 = new Form3();
        public Form1()
        {
            InitializeComponent();
        }
 
        private void button1_Click(object sender, EventArgs e)
        {
            f2.Show();
            f3.Show();
            timer1.Start();
        }
 
        private void timer1_Tick(object sender, EventArgs e)
        {
            if (f2.form2Closed)
            {
                timer1.Stop();
                f3.setText("Form2 Closed");
                f2.form2Closed = false;
            }
        }
    }
}
 

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
 
namespace Ventanas
{
    public partial class Form2 : Form
    {
        public bool form2Closed = false;
        public Form2()
        {
            InitializeComponent();
        }
 
        private void Form2_FormClosed(object sender, FormClosedEventArgs e)
        {
            form2Closed = true;
        }
    }
}
 

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
 
namespace Ventanas
{
    public partial class Form3 : Form
    {
        public Form3()
        {
            InitializeComponent();
        }
        public void setText(string value)
        {
            label1.Text = value;
        }
    }
}