Cassidy Helps Nicky Get Sober
It could be possible that the introduction of Cassidy's character is simply a means to help Nicky get to the place we seem him in the flash-forward. The last time we see Nicky in season three, he is still struggling with his alcoholism and hasn't yet come to terms with his PTSD from Vietnam. Perhaps Nicky and Cassidy will join forces to help each other work through their issues. She could even be the key to helping Nicky mend his relationship with his family, just like she wants to mend her relationship with her husband and son.