Complex sentence: After the death of her husband, Sophie became mentally unstable which frightens Martin, he sees his mother talk to an imaginary "friend" causing him to have sleepless nights and fall asleep in class at school.
Split into simpler sentences: After the death of her husband, Sophie became mentally unstable which frightens Martin. He sees his mother talk to an imaginary "friend" causing him to have sleepless nights and fall asleep in class at school.