I cannot think of any single word that means a person's life changed for the worse. While there are many phrases, i.e. fallen heroes for good guy turned bad, or take a turn for the worse, turning to the dark side,I cannot think of a single word that describes this phenomenon. An example would be in my subjective opinion, how Lindsey Lohan a child star took drugs as she grew older or Harvey becoming Two-face in Batman.
The sentence structure should be: He/She __________. And they would get the idea that their life turned bad or took a change for the worse.