Is copying pornographic data from a friend's disc without his knowledge considered theft, and what is the way to expiate this sin?

Downloading and viewing pornographic data is forbidden (haram), and it is not permissible to access the data owner's information without his permission. It would have been more appropriate to delete that data after viewing it, or to advise its owner of its prohibition. The current obligation is to delete it from the device, regret the action, resolve not to return to it, increase seeking forgiveness and performing good deeds, and advise the owner of the prohibition of viewing forbidden things.