I really don't know the meaning of the sentence. However, I let you know what I understand:
"She had gone to extremes to avoid seeing him."
I guess that she went as far as possible so that she couldn't see him any more.
It means that she did anything she could do to avoid seeing him. She may even have engaged in "extreme" behavior (such as staying home for weeks, avoiding people and places she associated with hiim), all because she didn't want to see him again.
One of my girl friends is a little fat and she is determined to lose weight by 15 pounds. She works out a lot and only eats a little everyday. She is longing to have a slim figure so much. I am really worried about her.
In this situation, I told her"Honey, don't go to extremes. You are gonna kill yourself." Is "go to extremes" here used correctly?