What is the solution in the book Wintergirls?

If by "solution" you also mean the resolution of the plot, we should first address the conflict that drives Wintergirls. 

Lia Overbrook has recently lost her best friend Cassie to bulimia. For years, Lia and Cassie have shared everything- even the fact that both of them struggle with an eating disorder. Now that Cassie is dead, Lia is alone in her struggle, grieving for her friend, and is wracked with guilt. In the hours leading up to her death, Cassie called Lia on her cellphone, but Lia never picked up. Now she worries that if she had answered the phone, her friend would not have died. 

Lia is haunted by her memories of Cassie and begins to see, feel, and smell her ghost. (There is some ambiguity as to whether Cassie's presence is supernatural or imagined as a result of starvation.) As months go on, Lia begins to unravel the story of what really happened in Cassie's final hours and her own eating behavior becomes worse.

The climax and resolution occur very quickly in this story, with Lia finding her way to the hotel room where Cassie died. There, Lia attempts to commit suicide by overdosing on medication. When she awakes, she realizes that she does not want to die and does not want to continue starving herself. Lia realizes that she has an opportunity Cassie did not: the opportunity to get better because she wants to. 

In short, the "solution" or resolution of the conflict was for Lia to visit the scene of Cassie's death and have her own near-death experience. It was only when she risked truly losing her life that she realized she did not want to die by her own actions and could get better. In a way, Lia's determination to recover is not in spite of Cassie, but in honor of her.