There is no one definitive answer to this question. Depending on your level of technical expertise, there are a few different ways you might be able to control your friend's computer screen. One way would be to remotely access their computer using a remote desktop application like TeamViewer or Splashtop. This would require that you have your friend's permission to access their computer, and they would need to provide you with their login credentials. Another way to control your friend's computer screen would be to physically connect your computer to theirs using an HDMI cable. This would allow you to mirror your computer's display on your friend's screen.