What should the questioner do for Allah to forgive her and her teacher, who has passed away, after they had an illicit physical relationship?

You can supplicate and seek forgiveness for this man, but the most important thing is sincere repentance: by abandoning the sin, feeling remorse, resolving not to return to it, avoiding the causes of disobedience, cutting off paths that lead to it, eradicating the root of evil, and being cautious of following the footsteps of Satan, while concealing yourself and not openly committing sins, and striving in righteous deeds.